The main difference is how the focussing screen is held in place, and whether you, the user can change it easily or not.
In the 500 C, the focussing screen and the Fresnel lens below are placed on a blade spring. This spring is pushing both up.
Over this is placed a metal frame which is screwed down, one screw in ach corner. These screws, how far they are screwed down, determine the correct position of the focussing screen.
Changing focussing screen thus involves not only finding a replacement screen, but also carefully calibrating the corect position of the screen, such that focus on film and in viewfinder correspond.
In the 500 C/M, the Fresnel Lens and focussing screen are sandwiched inside a double metal frame.
This 'package' is placed on four studs, and held in place by two movable clips on left and right sides. The position/height of the studs is set in the factory, and determine correct focus.
This way, you can change screen without having to calibrate its position.
You simply move aside the two clips, and let the screen drop out. Then place the replacement screen on the studs, and when you slide the viewfinder on again, the clips move in, holding the screen firmly in place.
There are a few more (most are non-apparent, and for users not very important) changes, which were made during the 'life-span' of both 500 C and C/M.