This lens was made 1943, year S. Rachmaninov died. It came with custom-made Zenit M39 mount,
it was an Exakta mount lens I guess and the lens in the photo is attached with M39->EF adapter.
Canon EF's register is 44.0mm and Zenit M39's 45.2mm, but M39->EF's flange thickness is now
1.85mm. It means that the original custom made brass M39 mount is too thick for Zenit camera
by 0.65mm. Of course my M39->EF adapter was ground to reach infinity focusing on my EOS 5D.
Although the above argument is based on that my EOS_5D is perfect condition for focus accuracy
and also its flange (back focus) thickness is accurate. Anyway I can manage this faulty Zenit M39.