Henry Blake: Klinger! It's four o'clock in the afternoon and you're still in a housecoat? Put on a dress! You never know who might be coming around.
Frank: [after Hawkeye hits him] Captain Pierce, I accuse you of striking a superior officer. A court-martial offense under the Articles of War. And I have two witnesses!
Trapper: [to Hawkeye] I didn't see anything, did you?
Hawkeye: My back was turned. [Drops towel in Frank's face.] BA-DUM-BUMP!
Trapper: Case dismissed.
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 ★