[Ian C]

The film is obviously slightly out of position at the ends (not in register with the film rails). Lack of normal tension in the film strip might explain this. That would allow the film to belly out from convex surface of film positioning rails near the retaining rollers just outside of the ends of the frame. This makes sense because that is where the film makes a sudden transition from the large concave radius (of the emulsion side) to the much smaller convex radius as it is forced under the rollers.

I think that the film rewind knob likely needs more resistance to ensure that the film has normal tension.

A technician might be able to adjust the camera to restore normal resistance to the movement of the film so that the film strip has the proper tension to keep it in register with the convex guide rails. I base my comments on the open-back illustration here.

http://manualcamera.info/widelux.htm