I've recently taken on rebuilding Five "unusual" Sonnars. It's my guess that some are Russian assembled from German parts, and others are Russian copies from the German designs. One is just strange. I've posted two so far, will be putting up three more.
The ZK lenses are reputed to be very good. I had one 1950 J-3 that was just "wrong" optically. The mount was much too long, the focal length was off.