Continuity mistake: In the Operating Room, as Charles hears that his phonograph has been "donated" for use during the emergency, he turns his head from left to right. In the close up, he lifts his head.
Continuity mistake: When the bomb disposal sergeant is sitting down at the end of the episode, he's facing the table. When the camera cuts in closer, his left shoulder is facing the table.
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 ★