Continuity mistake: As Charles talks to Finch, he hangs up the clipboard, and puts the pencil in his pocket. Then as he turns Finch to walk out, he puts the pencil in his pocket again.
Revealing mistake: In the closing scene, as the camera looks at the collected crowd, the camo net over the building poorly hides the edge of the "sky" background and the sound stage walls.
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 ★