The M Visoflex is 41mm, but the body adds 27.8mm. So the Visoflex lens is 68.8mm from the film. R lenses are 47mm from the film; you can't mount an R lens to a Visoflex and focus to infinity. If your question is for the M240, with direct use of the R lenses without a Visoflex, the adapter is 19.2mm thick. Leica now makes this adapter and Novoflex surely does too.