What you are not taking into account is the linkage of the rangefinder to the lens may be bent or out of whack or the back film gate may be different or the lens to film may be different, wear n tear over the years.
What matter most is how it actually focuses at the film gate and aligns with the range finder. If the rangefinder aligns, and the film image is sharp... everything else moot.
If youy want that to be the same on both cameras, well you'll have to spend a fortune to have someone like me to rebuild the camera completely piece by piece so everything is perfecly aligned as good as new. How old is that camera?
It amazes me how people compare light meters and get all bothered about being off a stop. It's all relative and means nothnig in the real world as long as it is consistant. You are not surveying to teh tenth of an inch or leveling dynamos with it, you mearly want sharp pictures.... end result.
It's not the fault of the reapirers, it's the pickyness of the user. 6" off is nothing, make it up in DOF.