The door may very well may have come from a stored or scrapped
US 73's in the desert, where spare parts are usually more economical then a new part.
Just my guess...
This practice has been done with cars for decades (how many different color fenders, hoods, trunk lids, etc. have you seen on cars?) why not aircraft?
"TransEastern! You'll feel like you've never left the ground because we treat you like dirt!" SNL Parady ad circa 1981