I bought a "like new" M6 TTL last week (serial number denotes a 1999 model) with only three rolls run through it and have fallen head over heels in love with it. Problem is, the 50mm Summarit that I've been using on my M8 will not mount correctly to it. The lens will fit into the mount but will not twist and lock. The issue is particularly strange in that the lens mounts and focuses perfectly on my M8 and Voigtlander R4a as well as my friends' M3's, M2's, and M4's. Furthermore, all lenses that I've used on my M6; 50mm Summilux, 50mm Summicron, 35mm Summicron, 75mm Summilux, an older collapsible 50mm and even my voigtlander, zeiss and konica lenses mount and focus perfectly.
I suppose I should mention that the Leica hood that I bought for the Summarit fit's a tad crooked (very slightly diagonal to the horizontal body lines on whatever camera I've mounted it to) but I can't imagine this having anything to do with the issue I'm having with the M6 as it has mounted just fine to other cameras, crooked hood or not. I know every company makes mistakes some times but this is really bugging me.