Other mistake: When Henry is trying to convince Baker how wonderful the camp is, Henry says that they "had 'The Thing' and 'The Blob' both in one week." The movie "The Blob" was made in 1958. This season of M*A*S*H is set in 1951 and 1952 (and the war ended in 1953). (00:03:55)
Revealing mistake: When Baker is talking with Father Mulcahy it is clear that Baker's nose is prosthetic. The colour doesn't match as well as it does in other scenes. (00:05:45)
Answer: The C-42 was a military variant of the Douglas DC-2. Very few C-42's were built, so it's questionable that Potter would specifically have seen that particular model, but, given his military background, it's not entirely unreasonable that he might use the military designation even when the aircraft in question is actually a civilian DC-2.
Tailkinker ★