The original Summicron LTM/M from 1953 was a 7-elements/6-groups design then changed to a 6/4 formula and again to an 8/5 formula for the aspherical version. There are also an APO (apochromatic) version, but I'll stick with the R version I had.
|Leica Summicron R 50mm F2|
I used it firstly with my Leica R6 and then as a mid-telephoto lens on a Sigma SD15 camera (APS sized sensor). The combination of the Foveon sensor and the "Cron 50" was a killer. My SD15 was converted from the Sigma SA mount to the Leica R mount by installing a custom-made mount from Sigmacumlaude.com.
I also used it for a while with the Pentax K5-II, by changing the lens R mount for a Pentax K mount from Leitax.
I sold this lens some years ago. Today I wonder how would it perform with my Sony A7-II...