There's an actor named Alex Henteloff who's a virtual deadringer for Stuart Pankin. Henteloff played the fake doctor in that 'MAS*H' episode, not Pankin. (Check Henteloff's page here on IMDb, and I'm sure you'll find it listed.)
I can understand how the two could get mixed. Their looks and body types are almost identical, they're roughly the same age, and they generally play the same types of characters. I remember seeing Henteloff on an episode of 'Night Court' and initially thinking it was Pankin.