The Price of Tomato Juice - S4-E15
Other mistake: Actor Jeff Maxwell plays Private Igor Straminsky, however when Maj. Burns is in line and asks for his name, he replies "Maxwell" and Burns in turn repeats the name "Maxwell", instead of "Straminsky."
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 ★