Horseman roll film holder jammed, help please

[ghart]

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!