Let's put this one to rest for good.
If there still was any doubt about Tom Mix's car being a Don Lee Cadillac, here is a (smallish) picture of a 1920 Cadillac by Don Lee for starlet Ann May. Unlike the Mix car, it has disc wheels and and a side spare wheel, but in the exact style of another Don Lee car given by Joao as an example. But the fenders are the same, and it even seems to have the same small lantern between the front and rear doors.