presume the rest of the roll has finished pictures as well, all with this VERY evenly spaced banding of over- and under-exposure, the banding reaching to the edge of the film.
You're right, it didn't come through the film gate from the lens, but how could it have come through winding the film? Do you, does anyone, wind a frame of film 10-evenly spaced distances for each frame?
But they are not evenly spaced I've put a venier caliper on my phone display, this means e.g. it might be leakage due to operating the wind on knob in small steps or variations of side force on knob.
On my phone it looks more like fogging even the variation in density...