April Fools - S8-E25
Visible crew/equipment: After the opening credits, in the closeup of Charles and Colonel Potter just as Charles complains about what's undoubtedly awaiting them in the mess tent, there's a shadow of a moving boom mic at the top, left corner of the curtain.
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 ★