I don't think it is the narrow stripe that is the problem, but rather the rest of the image which seems to have been exposed to 'false' light.
A while back I saw something similar on the negs from my M2 which I ended up concluding was caused by the change of lenses in bright light.
Try to check to see if you have switched lenses between the problematic shot and the following, and perhaps if you can remember if there was bright lighting when you changed lens.
It may be remedied by improving the light sealing by the shutter, which is not that good on a M2. I personally never bothered - I just take care about changing lenses in the shade.