In general, the film indexing is handled in the back. There is an interlock - a tubular protrusion with a pin inside that couples from the back to the body to "communicate" film wind status. There could possibly be dirt or a bent pin in that interface. Trying another back would be the next step. If the problem follows the back, a CLA of the back ought to fix things. Unfortunately, a CLA might cost more than the back -- although another back could be just another source of problems.
My first back (on an SQ-A) just happily wound the entire roll of film through without stopping. They are nice cameras, but the youngest are 15 to 20 years old, and they are quite complex mechanically.
Last edited by DWThomas; 12-21-2013 at 02:42 PM. Click to view previous post history.