Would it be possible that you stored the film next to a chemical for quite some time that would desensitise the emulsion at the edges of the the first few frames? A bit like a fat roll light leak but vapour, not light. I realise this is a long shot, but long term storage next to Ferricyanide etc? Or, frozen storage with humidity, ice forming, backing paper sticking... Brain storming.

It is puzzling how regular the top and bottom is. I suppose mechanical issue is more likely, in the camera.

Ps. I did not realise it is between frames, was thinking top and bottom. Ignore my suggestion, please.