The rear element of the M39 Helios protrudes about 3mm from the end of the mount thread when focused to infinity so no problems there on my K100D.
I've learned that Leica M39 has register distance of 28.8mm and Zenit M39 has exactly the same register distance as the Pentax/Praktica M42 (45.46mm). The thread on the Leica M39 is also of different pitch (M39x26tpi vs Zenits M39x1mm). I googled a bit and found a third M39 mount which is Braun Paxette M39. It has the same M39x1 thread as the Zenit but register distance is only 44mm so no infinity focus with this one.
edit: I quess the difference of the thread pitch i neglible because the adapters are sold as Leica and M39 compatible
Last edited by ovim; 05-02-2009 at 10:34 PM.