Amateur video taken at the 2012 Comic Con is San Diego CA, at a panel salute to TV animation's Filmation Studios. Featured was a special screening of a Marx Brothers pilot episode cartoon, "A Day at the Horse Opera," starring the voices of Pat Harrington, Ted Knight and Joe Besser.
You can read a little more about the toon in this site's Three Stooges TV Guest Appearances
Entire video; Marx toon @ 48:00
Or, you can shortcut to the section with the Marx cartoon by clicking this link