As it is well out of focus I suspect there's some hair, or thread hanging in the light path inside your M6. I suggest try to set the shutter speed to B, and without the film, open the back, press the shutter and look inside against the strong light. You should be able to see it. Good luck.
Contax G1, G2, 21/2.8 Biogon G, 35/2.0 Planar G, 90/2.8 SDonnar G, Nikon F4, 180/2.8 Nikkor,