Sorry to bump an old thread, but I'm brand new here and just wanted to chime in on this.
I have several rolls of "Svema 125" that I got as freebies from FPP. I home process, so I can tell you for certain that these are hand-loaded, used commercial film cassettes. They tape onto the tail of the film that was originally in the can, and bulk roll it like a reusable cassette. The light leaks you've mentioned only seem to make an appearance on the first one or two frames of the examples I've seen, with the film before it being completely overexposed, so I think that leakage is the result of their hand loading from bulk rolls. Personally, I like the effect it has. As it has no pre-exposed edge markings, I also like it for rolling onto 120 and 620 for my MF cameras.