It's perplexing.
On the one hand there's some overexposure. But note the portion of sky on the right of the fogging appears to have received *less* exposure than the remainder.
By itself this would suggest the first curtain is running just a tad slow and tapering the exposure near the end of its run. But that flies in the face of a second curtain sticking as suggested above, because, this will result in *more* exposure—not less.
I think there are a couple of things going on. It's a problem which is impossible to accurately diagnose with certainty online. You'd need to put the shutter on a tester to work out what each curtain is doing, to be sure of the remedy. If pushed, I'd hazard a guess that the first curtain is dragging a bit near the end of the film gate (or simply running slightly slower than ideal, enabling the second curtain to reduce the desired slit width). And that perhaps the curtains are not correctly capping as the mechanism is being wound. Don't forget that, like nearly every other focal plane shutter made in the last ninety-odd years, the Leica is "self-capping". Meaning it must achieve two distinct functions. The first, obviously, being to achieve correct exposure at its various rated speeds. But just as importantly it should adequately shield the film gate from light passing through the lens *as the blinds are being drawn*, as well as when the shutter is cocked, or released.
The strip of fogging visible *might* be due to a running irregularity in the curtains, that's true. But the presence of what seems to be a little underexposure next to the gate end means that, if all the shutters ailments are occurring as the blinds run—you seem to have a combination of both over *and* under exposure taking place, together. Whilst not impossible this is less common than the exposure simply tapering over or under.
Hence I'm pondering if perhaps the lath clearance is inconsistent as the shutter is cocked and the capping is inadequate for a few mm. Rick's suggestion of a film remnant creating problems might be on the money, because if something is fouling one of the curtain ribbons and making the curtain action uneven, I think it's plausible this might separate the laths momentarily during the cocking, and you'd then get something like what we see above—a basically correctly exposed frame with a fogging band superimposed over a seemingly random strip of the image.
The above is mostly conjecture, but offers a few possibilities for you to investigate. The Barnack Leica design doesn't lend itself to easy diagnosis without extracting the mechanism from the body, of course, so that's easier said, than done...
Cheers
Brett
