Other mistake: It's 1952 when Potter arrives at the 4077. He tells Radar that he joined the army during WWI when he was 15 (he lied about his age). That would mean that Potter was 50 when he arrives. But he said that he'd be retiring in 18 months. Also, in the episode "Point of View" Potter states that this was the first time in 37 years that he missed calling Mildred for their anniversary. That would mean that Potter was 13 when he got married.
Character mistake: When Frank is berating Igor about the food on the trays, he calls him "Sergeant." Igor has always been a private.
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 ★