My camera, an M2 with perfect looking shutters, which were replaced around 2000 by Leica in London, if I can believe the former owner.
I don't use a bulk loader, I load cassettes in a dark bag. I load Nikkor reels in a dark bag also. The striations are about half of the roll starting at one end, and fading out somewhere in the middle. That is the 2 images I show in the OP the worst frame and the best frame. See the lines in the worst frame go from frame to frame even through the dark area between frames. Would not that rule out curtains as a fault?
We will see on the next roll which is just about depleted, 5222. I will also use a different developer. If it shows up on that one, it may well be shutters, or mechanical.
I'm betting it is the film emulsion, or some handling defect that I inflicted on that roll.