It says what it is on the camera...
Mind you, they didn't change the name until some (short) time after they changed the camera.
So there indeed are 500 C cameras that are the same as 500 C/M cameras, except for what it says on the camera.
Look at the focussing screen. Are there two clips left and right, holding it in place? If so, it's a 500 C/M (though if it also says 500 C on the camera, it's not a 500 C/M .

).
The crank/knob itself is not a good guide. They are user-changeable, and you can put the a later style crank on an old camera, and vice versa, and old knob on a newer camera.