Anatomy

As with the FEDs and the Zorkis, the shell of Zenit "1", Zenit-S and Zenit 3 can be separated by removing a few screws, allowing to let the shutter assembly out (do the separation horizontally, to avoid losing the film pressure plate and it's leaf springs). Here, the top cover is removed, but could have been left, all screws are accessible from outside.

The mechanism of the mirror of Zenit "1".
A set of levers actuated by a screw on the axis of the drive coil pushes the mirror down while cocking, an arm holds it down till releasing the shutter.

The mechanism of the mirror of Zenit-S and 3.
At left, the arm that holds the mirror down before releasing the shutter, at right the cord used to lower the mirror during cocking.
The curtain is sewn around the vertical metal bar, while on newer cameras it is stuck in sandwich in this bar.

Seen from the other side: the curtain has its rubberized side towards the film, therefore it winds tissue inward, unlike in the more modern models.
The FED 1's curtain is assembled the same way.