Horseman roll film holder jammed, help please

One of my Horseman 6x9 roll film holders is jammed. The window shows "S", and the film advance and release levers cannot be moved, other than very slightly. I think what happened is that the back was opened and the film removed after the initial wind-on, before any frames had been taken.

It would be possible for me to take the top off the holder—I have done this with the Singer/Graflex holders, to service them, but if anyone has ideas or expertise that could fix the problem without surgery, I would be grateful!