Adam's Ribs - S3-E11
Revealing mistake: While Hawkeye's in the mess tent leading the protest shouting "we want something else" he climbs the tent pole, and in two of the wideshots the camera has panned a bit too far to the right, and we can see the edge of the tent facade set at the top right corner of the screen.
Adam's Ribs - S3-E11
Revealing mistake: As Radar, Henry, Klinger and Trapper get up to answer the emergency call, the outside background can be seen in the upper right corner of the shot, showing it was done on a sound stage.
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 ★