The Soviets believed in incremental improvements. Each model of whatever had only a few improvements or new features over the previous model. That way they could reduce the chances of a total failure in design or manufacturing.
There were two series or product development lines for the Zorki, one was the Zorki-1, Zorki-2, Zorki-3 and Zorki-4 and Zorki-4K. Removeable backs started with the Zorki-3.
The other was the Zorki-1, Zorki-2, Zorki-S (C), Zorki-2S (2C), Zorki-5 and Zorki-6. This was a bottom-loading design until the last improved version, which was the Zorki-6. The two product lines had a common ancestry, but they diverged after the Zorki-2.
-Paul