Hello,
I had a similar light leak with my M 2. I sent it to the Leica Customer service together with sample negatives. They charged me a lot of money, allegedly had exchanged the 2 curtains, but the leak was still there. Then I examined the edges of the curtains under a stereomicroscope and discovered a very tiny textile fibre particle between the curtain edges exactly where the leak was. I think the light spot came during transport and cocking of the shutter. I removed it carefully with a pair of tweezers and the problem was solved.