Here is the Zenit, first of the name. The idea is simple: adapt a mirror chamber on a Zorki. The base is the same, a lever pushes the mirror down when cocking, another holds it in place until the release of the shutter (you see nothing in the viewfinder when the shutter is not cocked, it is not yet the "instant return mirror", which will appear on the Zenit E at end 1967). This system makes that the camera is as little noisy as the Zorki. And very compact.
On the underside of the mirror housing, a small flap can be unfolded to stabilize the camera when placed on a flat surface. This tab has not been kept on the following models, what a pity.