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The main difference between the 501C and the 501CM is the gliding mirror system which is now also used on the 501CM.
Gliding mirror does not show vignetting with lenses having a focal length of 150mm or longer on the top of the screen.
There might also be some changes (improvment) which we do not know and can not see.
So if you intend to buy one of the two models i suggest that you go for the 501CM.