Continuity mistake: Just after the announcement that General MacArthur is coming, Trapper puts on his sunglasses, then the shot switches to Henry walking, and in the background Trapper is putting his glasses on again.
Visible crew/equipment: As Col. Whiteman leave Sentry Post 1, where Klinger is, the boom mic is reflected on his helmet.
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 ★