This might be very interesting for...

[LucisPictor]

...all those guys who shoot with a M39 rangefinder:

Click here to see on Ebay

As far as I see that correctly, this device enables you to use M42 lenses on a M39 rangefinder cam.
I don't know how you would focus, but there is a scale on this device.
Perhaps set the lens to infinity and focus with the scale???

[kds315*]

Nope, sorry to correct - this is NOT to mount M42 lenses on M39 Leica thread mount, only for a special lens head of a certain focal lenghth (135mm if I remeber correctly) without own focusing!!

If you want to mount standard M42 lenses onto LTM (Leica Thread Mount M39), all it needs is a simple adaptor ring which bridges the different registers: M42 - 45.5mm, LTM - 28.8mm. There is a Leica Ring which does that, and also some russian sellers on ebay selling that cheaply.
There is NO RANGEFINDER coupling of course.

[F16SUNSHINE]

I bought this one for C/Y to M mount.
Click here to see on Ebay

It has no coupling either but does get infinity and the focus scale is accurate on the lens.
The only reason I got it is because no other bids came in after my opening bid. $25 was a lot less then the $130 these usually go for.
I will try and make some comparisons to show the advantage of non retro focus vs. retro focus lenses.
It will be interesting to see for example how the Biogon 2/35 compares to the Distagon 1.4/35.
Two lenses that are clearly at the top of their FL field.
Sure we can do this with two cameras as well. Having both lenses draw onto the same sensor or roll of film is more accurate for comparison.

[LucisPictor]