I noticed the same is true on my Type 2 Rigid Summicron. The Leica Rangefinder is calibrated for 51.6mm. The focal length of individual lenses may deviate slightly, and most manufacturers just leave the RF cam flat. The Leica cam is milled to take the deviation into account. By the looks of my RF Cam, the difference is about 0.1mm. The cam is milled to correct for the slight deviation in the focal length of this particular lens from 51.6mm. I suspect this was required when the minimum focus of Leica lenses was brought in to 0.7m from the old 1m standard of the M3 generation lenses.
Originally Posted by Dovaras