While I truly enjoy my screw mount Leicas, it took me a long time to find their SLR counterpart. While essentially it really shouldn’t have been that hard, once you think about it.
I know Leica built some truly terrific early Leicaflexes, but they are more related to the M series rangefinders than to the earlier screw mount cameras.
Same time, the Russians copied the early Leica II and went on from there to produce a Single Lens Reflex camera. Enter the Zenit line of SLR cameras from the early 1960s! Incredible fun for petty cash!