Did you ever have the small tank resting on its side during any stage of processing??
It no longer sounds like a camera problem--from what we can see of the banding it seems too descrete and uniform to be a light leak and a leaf shutter could not cause this either. It really looks as if the tank was resting on its side at some point during processing. It wouldn't take much time to cause this if the film was on its side and not completely covered at all times.?