Other mistake: When the scene is of the cow in the barn, the background hilly area can be a painted backdrop.
Other mistake: Looking at the pins when Margaret bowls at the end, the 5 pin slides backwards towards the camera, but not fast enough to bounce off anything and then fly in from off screen. Granted this was done in slow motion, but the physics don't seem to add up.
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 ★