First, numberless Zenit, also sometimes called as "Zenit-1". Produced 39.019 pcs in 1952 - 1956. Camera in pictures is a 1954 and with wrong, later Industar-50 lens. Correct lens is Industar-22 with focus lever up to 1955. Prototypes and preseries had slightly different top plate or name plate. Based on Zorki rangefinder camera, bottom film loading. Cover vulcanite, not leatherette.
I've added a picture with Industar 22 lens (napchop)
Differences to Zenit-S:
Mirror is returned with film advance after picture taking.
No flash sync.
Rewind control lever instead of collar.
Shutter speed knob is full, no center index mark. Speeds 1/25, 1/50, 1/100, 1/250, 1/500 and B.
Earliest known Zenit-1 with s/n 520019 documented by Alexander Schulz in his book "Zenit - Die Geschichte der russischen Spiegelreflex-Prismensucherkamera mit M39-Objektivanschluss" (german) to be found at: http://www.g-st.ch/privat/kameras/zenitbuch.html