Continuity mistake: While Colonel Potter is painting the portrait of Charles, the portable military field desks behind Potter are all open - note the front doors are flipped down, but in the closeup of Charles holding the painting of himself the doors are folded up. And the books, etc. atop the field desks change as well.
Continuity mistake: While Charles is reading the letter about his nephew being discharged because of fainting spells, Klinger feigns a fainting spell falling forward onto Hawkeye's cot, and his body lies lopsided with his leg hanging off, but when it cuts to the next shot Klinger's perfectly positioned on the cot. Also, note the mail Klinger tossed earlier onto Hawkeye's cot, near his pillow, is now neatly tucked under the blanket towards the foot of the cot.
Continuity mistake: In the last O.R. of the episode, Winchester is wearing a surgical gown with the same blood stain pattern as in the previous episode. This also would be impossible given the cleaning procedures.
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 ★