Shows back part of the Leica Summicron 50mm's, the 1998 model to the left, 1973 model to the right. Notice the different set-up of the bronze-like cylinders, which travels back and forth when focusing (its the inner cylinder in case of the 1998 model). There is a 3mm difference of diameter between these two cylinders. So in effect the 1973 cylinder has a 1.5mm wider contact point on the "push arm" mechanism on the camera (the "roller" on the arm in the camera that moves the rangefinder ), as compared to the 1998 model. This "roller" is 3mm size (at least on the Epson R-D1 camera, and ASAIK also the Leica M8), and contacts towards the cylinder - so the 1.5mm difference between the two lenses just *might* be enough to create a mechanical failure.