[I can't believe there is not already a thread for this, but I couldn't find one.]
This thread is for Leica R lenses that have been modified for use on Pentax bodies. Leica has discontinued this mount in favour of the M-mount, so in theory these lenses will only get more valuable as people seek them out for conversion.
There were 50 or so Leica lenses, not counting those from third-parties, made for R mount over the years. So lots of choice! Here's what mine looks like in use.
Leica Macro-Elmarit-R 60mm on the Pentax K20D