Quote Originally Posted by Robert Kennedy
I may be thinking of something else. But I swear that I saw somewhere on R. Monaghan's site mention of using R mounts on M cameras or vice versa.

Then again, I have been taking pain killers....
Robert, the Bessa "R" cameras are rangefinder cameras. The Bessa "R" lenses are designed specifically for use with rangefinder cameras. In general, you can use Bessa "R" lenses on the M (Leica rangefinder bayonet mount) cameras (exceptions are "R" lenses with Nikon or Contax rangefinder camera mounts).

The problem occurs when you try to use a single lens reflex (SLR) camera lens on one of these rangefinder cameras. The mirror chamber on a SLR (which is not present on a rangefinder camera) necessitates moving the SLR lens far enough away from the film plane so that there is room enough to accommodate the moving mirror and mirror mechanisms without mechanical interference from the lens.

Therefore, you have difficulty moving a SLR lens close enough to the range finder camera's film plane to focus it at infinity - without modifying the lens and/or the lens mount. The shorter the focal length of the lens, the bigger the problem.