Forgive this silliness, but I found a 500c and have been enjoying myself. I have got on fine with the two film backs I have used so far (each has the folding handle for winding on the film). KEH sent me two further backs today, but they are a different design - behind the ISO reminder is a hole to see the film's paper backing moving, and the wind on knob isn't a folding handle, but resembles the catch for removing the film carrier from the back. It is a steel ring, divided in two, and either half can fold out to allow the ring to be twisted and thus wind on the film. I tried to load one this evening and ruined a film - it wound all the way on without stopping at the first exposure. I couldn't manage to rewind the film onto the original spool correctly in a changing bag so I scrificed the film and wound it back in daylight onto the original spool and tried again with the same result. So I tried it in the other back and the same thing happened. I'm doing something wrong! Any idea what?