The Mir has been presented to the Public in about 1958 and it has the correct size and thread pitch to qualify for M39... just the wrong lens/film flange distance. Basically that Zenit is a Zorkii with a mirror added to the front... then they realized that 39mm arenŽt wide enough for many non-Retrofocus lens designs and widened it to Start bajonet mount and M42 (which was already in use by Praktica - but on Soviet pressure. Rumours are that they had an order on SLRs from the Soviets and worked hard to fulfill the schedule... there wasnŽt enough time to implement a bayonet mount, so the M42 design was chosen).
Originally Posted by titrisol
To use it with the Bessa or any other RF youŽd need a step up ring of 45,46mm minus 28,8mm... and machine it in tolerances of +/- 0.02mm.
Certainly not impossible, but a major hassle. YouŽll loose RF coupling, youŽll get an ealy non-retrofocus design... a J12 in M39 is better.. ;-)