Factual error: Out by The Gun, Hawkeye's Captain insignia on his garrison cap is missing the lower corner of one bar. So, it would be considered unserviceable, that is, unfit for use, and should have been replaced with a new insignia.
Factual error: General Clayton is wearing ribbons on his regular fatigues. This is incorrect as the ribbons would only be displayed on the dress uniforms. Only the officer's rank would be on the fatigues.
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 ★