I've been revising the Terrytoons filmography and there is one single short that isn't listed in any academic or official source except Jeff Lenburg's Encyclopedia of Animated Cartoons: a Hector Heathcote short supposedly released in May 1961, titled "Foxed by a Fox".

No other books mention it. Usually the Hector Heathcote shorts are listed as segments of the NBC Saturday show, "The Hector Heathcote Show" that ran from October 5, 1963 to September 25, 1965. 35 Hector Heathcote episodes are commonly listed along with 17 hashimoto episodes and 19 Sidney The Elephant episodes, adding up to a total of 71 segments in the show. 39 of the 71 segments were originally released theatrically, but later during the 1960s and 1970s, 21 of the made-for-TV episodes made their way to the big screen too, leaving only 11 segments that never got a theatrical release.

Couldn't find it on copyright records (but I could find the other 35 Hector Heathcote shorts). Gerald Hamonic's Terrytoons book also doesn't mention this short.

There are entries for it here (Intanibase) and in BCDB. There was a Terrytoons-fandom-wiki page for this short, with no further details and a faked title card that used the Comic Sans font.

Some fansites devoted to old-school Saturday morning animated series mention "Foxed by a Fox" in Hector Heathcote listings, making me suspect it may be an episode that was included only in syndication packages and wasn't originally released in the original NBC show:
https://oldskool.cartoon.../hectorheathcote1958.htm 
https://derecuerdos.blog...06/hector-heathcote.html  (in Spanish)
http://ft1978.altervista...-Z/I/ilprodeettorre.html  (in Italian)
Note that these webpages are relatively old and don't exactly replicate the Intanibase or BCDB listing.

I couldn't find video evidence.

There is a short with the same title, released in May 1955, directed by Connie Rasinski.

Do any of you have further evidence?

---

The Italian page is intriguing: they list 20 Hector Heathcote episodes ("Foxed by a Fox" among them) and previsously state in the text that there were "16 more" episodes produced, that "remain unreleased in Italy": "Sono stati prodotti anche 16 corti tra il 1958 ed il 1963 su questo personaggio, rimasti però inediti in Italia.". They mention Rai 1free-to-air channel.

---

The Encyclopedia of Animated Cartoon Series/The Encyclopedia of Animated Cartoons by Jeff Lenburg is not known for its accuracy. The plot summary for Snoopy Come Home in The Encyclopedia of Animated Cartoons (1991 edition) ends with Snoopy writing his last will and testament, then committing suicide!

I'm trying to get a definitive Terrytoons filmography.

In the past, I compared the following:
- the one from "Of Mice and Magic", by Leonard Maltin, that doesn't cover the post-1968 period;
- the original list compiled in 2014 in Wikipedia (https://en.wikipedia.org/w/index.php?title=List_of_Terrytoons_animated_shorts&oldid=621721964). This Wikipedia article was the base for subsequent changes in Wiki itself and in an article of a deleted Terrytoons Fandom/Wikia domain;
- the 1957-1971 filmography compiled by Charles Brubaker in the old DFE domain of the defunct GAC page (https://web.archive.org/web/20150508225706/http://dfe.goldenagecartoons.com/terrytoons60s.htm)
- the Box Office Magazine listings of the 20th Century Fox-Terrytoons shorts (for the late 1957-1971 period, comparing with Brubaker's listing)

The 1930-1956 period seems straightforward. There were date and series differences between the unsourced but strangely precise Wikipedia list and Maltin's list. There were some films whose series neither Wikipedia nor Maltin got right.

Comparing Maltin's list to Brubaker's, Maltin missed a bunch of films from the post-1956 period. Further comparing Brubaker to Box Office Magazine, there are films not dated to month-precision, that Box Office lists with month-precision: if we assume the press releases are true, from 1964 onwards the Terrytoons schedule was simply one film per month.

Now I'm adding a 5th source: Gerald Hamonic's filmography, from his GREAT Terrytoons book. Unfortunately (for my personal taste) the list is in alphabetical order.

I didn't compare the whole 1930-1956 period from Hamonic. That would be painstaking... I only skimmed through some films for which Wikipedia and Maltin list the wrong series, and I could see that Hamonic got the series right.

In the post-1956 period, Hamonic's listing has differing dates in several films. BUT... His list LOOKS like a definitive thing. He lists theatrical releases from the 1920s (silent Fables) until 1975 (all originally made for TV). I didn't know that Fox was still releasing Terrytoons theatrically that late, the common knowledge of all the other listings was that the releases ended in 1971. And Hamonic states: "until at least 1975", i.e., those are just the latest KNOWN releases, there could be more, undocumented or undiscovered.

---
EDIT: another feature that makes Hamonic's list seem definitive is that he wrote day-precise release dates for many films of the post-1956 period.
---

So, I didn't care for date differences and series differences. I focused on the more chaotic period, post-1956. Taking Hamonic's list as the primary reference, the following divergences/uncertainties persist:

- Brubaker's list states that the Deputy Dawg episode "Shotgun Shambles" (also registered as "Shot Gun Shambles") was theatrically released in 1962. I couldn't find any other evidence that this film was released theatrically, it isn't listed as theatrical by Hamonic, only TV;
- Contrary to all the other sources, the Box Office Magazine listings always mention the December 1962 release as being actually "A Fight to the Finish", the 1947 film, explicitly stating that it was the Mighty Mouse reissue, not the Hector Heathcote episode "A Flight to the Finish". Probably the magazine or the people at Fox mismatched the film title?? Every serious source lists the Heathcote title;
- Lenburg's Hector Heathcote episode list, only in the 3rd edition of his Animated Cartoons Encyclopedia, is still the only book that mentions the namesake of this thread, "Foxed by a Fox", but it's weirdly listed in obscure nostalgia fanpages that don't seem to have been sourced from Lenburg. A case for the existence of this episode is that the commonly listed segments of the HH Show amount to 71, only 1 segment away from 72, that would allow for a perfect schedule of 36 two-segment presentations (1 Heathcote + 1 Sidney or Hashimoto) or 24 three-segment presentations (12 presentations with 2 Heathcotes + 1 Sidney or Hashimoto and 12 presentations with 1 Heathcote + 1 Sidney + 1 Hashimoto, do the math!).

---

I didn't know thatFox was still releasing Terrytoons theatrically that late, the common knowledge of all the other listings was that the releases ended in 1971. And Hamonic states: "until at least 1975", i.e., those are just the latest KNOWN releases, there could be more, undocumented or undiscovered.

According to Of Mice and Magic, 20th Century Fox released Star Wars with a Terrytoon, though I think it was only overseas, not in the US.

- Brubaker's list states that the Deputy Dawg episode "Shotgun Shambles" (also registered as "Shot Gun Shambles") was theatrically released in 1962. I couldn't find any other evidence that this film was released theatrically, it isn't listed as theatrical by Hamonic, only TV.

Again, according to Of Mice and Magic, there were requests from theaters "especially in Texas" to release the Deputy Dawg shorts theatrically. Whether this actually happened, I have no idea.

According to Of Mice and Magic, 20th Century Fox released Star Wars with a Terrytoon, though I think it was only overseas, not in the US.

Again, according to Of Mice and Magic, there were requests from theaters "especially in Texas" to release the Deputy Dawg shorts theatrically. Whether this actually happened, I have no idea.

The question "has this Terrytoon been released theatrically?" is less relevant IMO than "does this cartoon exist?". We'll probably never know for sure the exact shorts ever released theatrically by 20th Century Fox/CBS-Terrytoons after the studio shifted its focus to television shows. Who knows if a random countryside theater ever screened a certain short?... For the record, Hamonic lists 9 Deputy Dawg episodes released theatrically, but "Shotgun Shambles" isn't among them. I found some comments in CartoonResearch.com blog mentioning "Shotgun..." among 6 theatrically released Deputy Dawg shorts, just like the Wikipedia article that mentions 6 theatrical releases (without listing them).

The "A Flight to the Finish"/"A Fight to the Finish" mismatch would be peculiar if true. Well, someone made the mismatch: the magazine, the studio, or the historians. There is the possibility that the studio offered the Mighty Mouse short, but ended up distributing the Hector Heathcote instead.

The most relevant uncertainty here is how "Foxed by a Fox" came to be... Whether it exists or not...

---

I'm trying to get a definitive Terrytoons filmography.

In the past, I compared the following:
- the one from "Of Mice and Magic", by Leonard Maltin, which doesn't cover the post-1968 period;Jerry Beck has mentioned over at his CartoonReaserch blog that the 1968 cut-off point was chosen mainly because that's the final year when shorts were originally produced for theaters, but I do find it unfortunate that still probably mix in shorts that were originally produced for TV.
- the original list compiled in 2014 in Wikipedia (https://en.wikipedia.org/w/index.php?title=List_of_Terrytoons_animated_shorts&oldid=621721964). This Wikipedia article was the base for subsequent changes in Wiki itself and in an article of a deleted Terrytoons Fandom/Wikia domain;This list seems to be a combination of Braubaker's GAC list and Of Mice and Magic
- the 1957-1971 filmography compiled by Charles Brubaker in the old DFE domain of the defunct GAC page (https://web.archive.org/web/20150508225706/http://dfe.goldenagecartoons.com/terrytoons60s.htm)Braubaker did state over at the Old GAC forum that he was manly interested in Post-Paul Terry Terrytoons thus this list doesn't include anything pre-1957.
- the Box Office Magazine listings of the 20th Century Fox-Terrytoons shorts (for the late 1957-1971 period, compared with Brubaker's listing)The Box Office has to be considered as a primary source or close to it, but primary sources might not always be 100% correct

The 1930-1956 period seems straightforward. There were date and series differences between the unsourced but strangely precise Wikipedia list and Maltin's list. There were some films whose series neither Wikipedia nor Maltin got right. Paul Terry's Terrytoons were exclusively produced for theaters originally, even if old BW Terrytoons had started to appear on TV as early as 1953. Paul Terry sold the Studio to CBS in 1955 and retired he left in January 1956, so it appears to be the reason why no Terrytoons were released between August 1956 and January 1957. To further highlight this Gene Deitch wasn't hired until June 1956.

Comparing Maltin's list to Brubaker's, Maltin missed a bunch of films from the post-1956 period. Further comparing Brubaker to Box Office Magazine, there are films not dated to month-precision, that Box Office lists with month-precision: if we assume the press releases are true, from 1964 onwards the Terrytoons schedule was simply one film per month.I think Box Office is most accurate here unless Maltin (Beck) or Brubaker had access to information that out weights the Box Office.

Now I'm adding a 5th source: Gerald Hamonic's filmography, from his GREAT Terrytoons book. Unfortunately (for my personal taste) the list is in alphabetical order.Filmographies in Alphabetical order should be banned as It makes everything so clustered and confusing

I didn't compare the whole 1930-1956 period from Hamonic. That would be painstaking... I only skimmed through some films for which Wikipedia and Maltin list the wrong series, and I could see that Hamonic got the series right.Going through Hamonic list will take longer than its needs to be considering its alphabetical order. So far I haven't yet found film from 1930-1956 Paul Terry's Terrytoons era that is not included in Maltin's I have compared all films from letter A to Z, and I will make a new post on those observations and discoveries.

In the post-1956 period, Hamonic's listing has differing dates in several films. BUT... His list LOOKS like a definitive thing. He lists theatrical releases from the 1920s (silent Fables) until 1975 (all originally made for TV). I didn't know that Fox was still releasing Terrytoons theatrically that late, the common knowledge of all the other listings was that the releases ended in 1971. And Hamonic states: "until at least 1975", i.e., those are just the latest KNOWN releases, there could be more, undocumented or undiscovered.It seems that Hemonic's filmography includes everything Paul Terry worked on and what his studio worked on after he retired, but this also another thing that makes Hemonic's list so painstaking to go through is that everything is in alphabetical order regardless of who the producer was or studio also worth pointing out Hemonic doesn't differentiate in the filmography differences between Paul Terry's single ownership and CBS ownership while Hemonic does differentiate between some of the earlier ownerships pre-1935, as for that 1975 date Jerry does confirms that over at cartoon-research that this was indeed the case with that said I have no idea why Braubaker ends his filmography in 1971 then.

---
EDIT: another feature that makes Hamonic's list seem definitive is that he wrote day-precise release dates for many films of the post-1956 period.Some of the pre-1957 shorts only have month and year so clear that more research is indeed needed into the exact release dates of the films.
---

So, I didn't care for date differences and series differences. I focused on the more chaotic period, post-1956. Taking Hamonic's list as the primary reference, the following divergences/uncertainties persist:

- Brubaker's list states that the Deputy Dawg episode "Shotgun Shambles" (also registered as "Shot Gun Shambles") was theatrically released in 1962. I couldn't find any other evidence that this film was released theatrically, it isn't listed as theatrical by Hamonic, only TV; Worth pointing out that Hemonic's filmography in its TV sections lists at least 8 or 9 Deputy Dawg cartoons as having been also released in theaters but Shotgun Shambles isn't amongst and I think I know the reason why, since as according to Jerry Beck over at cartoon-research CBS Terrytoons produced theatrical titles for every cartoon of Deputy Dawg series. So I wonder i Braubaker

- Contrary to all the other sources, the Box Office Magazine listings always mention the December 1962 release as being actually "A Fight to the Finish", the 1947 film, explicitly stating that it was the Mighty Mouse reissue, not the Hector Heathcote episode "A Flight to the Finish". Probably the magazine or the people at Fox mismatched the film title?? Every serious source lists the Heathcote title;Box Office might be wrong here as both cartoons exist one which is a Mighty Mouse title other is Hector Heathcote

- Lenburg's Hector Heathcote episode list, only in the 3rd edition of his Animated Cartoons Encyclopedia, is still the only book that mentions the namesake of this thread, "Foxed by a Fox", but it's weirdly listed in obscure nostalgia fanpages that don't seem to have been sourced from Lenburg. A case for the existence of this episode is that the commonly listed segments of the HH Show amount to 71, only 1 segment away from 72, that would allow for a perfect schedule of 36 two-segment presentations (1 Heathcote + 1 Sidney or Hashimoto) or 24 three-segment presentations (12 presentations with 2 Heathcotes + 1 Sidney or Hashimoto and 12 presentations with 1 Heathcote + 1 Sidney + 1 Hashimoto, do the math!).Lenburg is not known for accurate or correct information, Maltin and Hamonic mention Foxed by A Fox (1955), and its not Hector Heathcote cartoon abet not one which was released to theaters.

My responses are in bold above.

In my observations, I found that there were no differences between Maltin (Beck), Wikipedia, and Hemonic, when it comes to Paul Terry co-ownership/full ownership of the studio.
With that said it seems according to copyright registrations that only seven Cinemascope shorts were produced under Paul Terry's ownership:
AN IGLOO FOR TWO
GOOD DEED DAILY
BIRD SYMPHONY
LITTLE RED HEN
PARK AVENUE PUSSYCAT
URANIUM BLUES
SCOUTS TO THE RESCUE

The following Academy Ratio shorts were produced shorts were produced under Paul Terry's ownership and released after he retired:
THE CLOCKMAKER'S DOG
MIAMI MANIACS
HEP MOTHER HUBBARD
BAFFLING BUNNIES
PIRATE'S GOLD (re-copyrighted to CBS-Terrytoons)
A HARE-BREADTH FINISH (re-copyrighted to CBS-Terrytoons)
AFRICAN JUNGLE HUNT (re-copyrighted to CBS-Terrytoons)
DADDY'S LITTLE DARLING (re-copyrighted to CBS-Terrytoons)
LOVE IS BLIND (re-copyrighted to CBS-Terrytoons)

The following Cinemascope shorts released in 1956 all carried this copyright notice: Terrytoons A Division of CBS Television Sales Inc
OCEANS OF LOVE
POLICE DOGGED
THE BRAVE LITTLE BRAVE
CLOAK AND STAGGER
LUCKY DOG

Now for the differences between Maltin (Beck), Braubaker and Hemonic
The following are listed by Braubaker but not by Maltin (Beck) and not by Hemonic :
BANANA BINGE
MEAT, DRINK, AND BE MERRY
REALLY BIG ACT
CLOWN JEWELS
TREE SPREE
SHOTGUN SHAMBLES

The following are listed by Braubaker and by Hamonic but not by Maltin (Beck):
UNSUNG HERO
SAPPY NEW YEAR

Only listed by Braubaker:
SHOTGUN SHAMBLES

Only listed by Hamonic, Maltin (Beck) end their filmography in 1968.
THE SHRINKER (1969)
THE DRIFTER (1970)
WIND BAG (1971)

Braubaker ended his filmography in 1971 these are in Hemonic's filmography along with a handful of reissues of Heckle and Jeckle shorts.
1972:
THE INVISIBEAM
HIGH FLYER
THE FIRST TELEPHONE
HAPPY HOLLOW HAY RIDE 
THE TIMEKEEPER
MOVIE MAGIC
BARREL OF FUN
DON'T BURRO TROUBLE 
THE SCARECROW
HOKEY HOME MOVIES
TRESTLE HASSLE
THE TIME ERASER

1973:
OSCAR'S MOVING DAY
EXPERT EXPLORER
THE CHESTNUT NUT 
THE RETURN OF THE MONSTERIZER
SPACE COWBOY
PEACE PIE
PIRATE PLUNDER BLUNDER 
THE PAPER MONSTER
WHO'S DRAGON
THE HECTORMOBILE
HARM SWEET HOME 
THE SQUARE PLANET

1974:
HOLD THE FORT
THE RAVEN
JUNGLE JACK
VALLEY FORGE HERO
THE BIGGER DIGGER
MIXED UP MATADOR
HAR HAR HAPOON
HOBO HASSLE 

1975:
THE PLASTIC BLASTER
LI’L WHOOPER
MELVIN THE MAGNIFICENT
FRIEND FOX
THE JUNKER
DOG GONE CATFISH
THE MONSTERIZER
PEOPLE’S CHOICE

A case for the existence of this episode is that the commonly listed segments of the HH Show amount to 71, only 1 segment away from 72, that would allow for a perfect schedule of 36 two-segment presentations (1 Heathcote + 1 Sidney or Hashimoto) or 24 three-segment presentations (12 presentations with 2 Heathcotes + 1 Sidney or Hashimoto and 12 presentations with 1 Heathcote + 1 Sidney + 1 Hashimoto, do the math!).

Checking some Newspapers for Hector Heathcote, it appears that The Hector Heathcote Show used something similar to what you said. Some episode DO contain the 1 Heathcote + 1 Sidney + 1 Hashimoto (Klondike Strikes Out/Hashimoto San/Banana Binge - December 26, 1964), some contain 2 Heathcote + 1 Sidney or Hashimoto and + 1 random cartoon (Valley Forge Hero/Wind Bag/Honorable Family Problem/Stunt Men - February 20, 1965), some contain 1 Heathcote + 1 Sidney or Hashimoto and + 2 random cartoon (Land Grab/Really Big Act/The Wayward Hat/Fabolous Fireworks Family - May 29, 1965) and 1 Heathcote + 1 Sidney or Hashimoto and + 1 random cartoon (Crossing the Delaware/The Potter's Wheel/Sappy New Year - January 2, 1965). Due to that, it appears that it's less likely that adding another cartoon would allow for a much more perfect schedule (admittedly, I haven't done the math on this so I can't say if it WOULD, but from this, it appears less likely).