Not necessarily a flash of light can expose part of the film and not the whole reel. I still believe that it could be caused by reflections in the gate and these are dependant on the location of the lightsource. A C-mount lens should cover the film area without vignetting or any other problems. Can't explain the uneven devlopment since I doubt they used reels to develop the film.
Visualproducts sells a C mount to PL-Adapter to use PL-Lenses on C-mounts oh the possibilities.