I have a IIa , the film counter mechanism has a lever with a spring on the end, if this spring bit gets broken it jams the winding mechanism because it cannot be reset, there is a small spring loaded pawl in the film advance lever which stops the film being wound on (all part of the mechanism that stops you winding on when counter reaches zero} my counter spring broke and replacements are available but expensive, I think it was one of Chris Sherlocks tips, to tie back this feed pawl in the wind lever with something like dental floss, it means you dont have a film counter but makes the camera usable again, either advance the counter manually, or just keep a check on how many exposures you have made so you know when you are approaching end of film and wind on gently so as to not strip the teeth on the cocking rack.I did this with mine and it has worked perfectly ever since. Just keep an eye on how many frames you have shot.