Continuity mistake: When Frank walks into the Swamp and puts down the typewriter, in shots facing Frank the manila envelope's open flap faces up, but in shots facing Trapper and Hawkeye while they're pretending to argue the envelope is flipped over with the flap facing down, then when Frank walks back to the envelope the flap faces up.
Continuity mistake: While Hawkeye and Trapper are playing shotglass checkers Frank lathers shaving cream onto his face, but in his closeups the thick foam is dried out.
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 ★