Visible crew/equipment: When Klinger walks into Henry's office while Margaret and Frank are there, we can see the chalk T-mark on the floor behind Margaret where Radar was standing moments ago.
Visible crew/equipment: When Dr. Freedman's in Henry's office before Klinger walks in, just as Henry says, "You're the doctor," the shadow of the moving boom mic can be seen at the top left side of the screen.
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 ★