I don't know specifically about the 6, but a good rangefinder will typically have two adjustments - call them slope and intercept. If they're not both in alignment, you could easily have focus match between two bodies at one point but be off everywhere else.

You might try sending the bodies to these folks:

http://www.precisioncameraworks.com/

http://precisioncameraworks.com/Media/MamiyaInfo.pdf