Thanks, I'm glad to hear an answer to the riddle of the spots! 
The FSU lenses have a different back-focus specification than Leica standard. This results in accurate focus either at infinity or close-up, as the lens can be adjusted with internal shims to do either but not both. Longer lenses are more affected, with the 50mm just starting to show it. Perhaps you see it more on the M8 due to the higher resolution and sharpness...
The FSU lenses have a different back-focus specification than Leica standard. This results in accurate focus either at infinity or close-up, as the lens can be adjusted with internal shims to do either but not both. Longer lenses are more affected, with the 50mm just starting to show it. Perhaps you see it more on the M8 due to the higher resolution and sharpness...