daveymac-
If "your museum" is the Canadian Museum of Flight, the more detailed answer to your question, following up on Scott's info, is that your DC-3 was delivered to American Airlines on February 24, 1940 and was flown as "Flagship Texas." (The second Flagship Texas, by the way.) That history, as Scott said, would answer the question as to why your aircraft has a right hand entry door. American president C.R. Smith, who talked Donald Douglas into building the DC-3, wanted his airline's passengers to enter and exit the aircraft without seeing the luggage operations, which took place on the left hand side of the aircraft. Thus the right hand entry doors. Made for a classier entrance/exit for the passengers in C.R.'s mind. The DC-3 at Lonestar in Continental Airlines markings, FYI, also has the right hand door as it is another original American Airlines Flagship, which flew as "Flagship Big Springs." It was delivered just a couple of months after your DC-3, in May of 1940. But I digress.
kevin