Carl Zeiss Jena 5cm F2 Sonnar, 1934, converted to Leica mount on the M Monochrom. I did this conversion.
The new British company Skyllaney is now offering this service.
Same lens, while it was still in Contax RF mount- on the Contax IIIa. I've had this lens for a long time. Bought it with a Nikon M (nosync) at an antique store. It was cheap...