Nothing in the sprocket holes, it is restricted to between the 24mm part of the film. It also occurs between exposed frames, ie, a as a vertical burn between frames, again not exceeding the 24mm. As such Im kind of hedging towards the internal leak from the rangefinder, maybe up behind the dx strip agian, and probably occuring during film rewind. A back door leak would be tapered from the mask I think too. The day I have frig around using a leather case to fix something that Leica havent put together properly will be the day there is a bunch of cheap recent CLA kit on eBay.
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Of course there is plenty more.