The "professional" cameras: Production

Along with the mass market cameras, KMZ produced between 1958 and 68 "professional" cameras, which are distinguished by interchangeable viewfinders ( waist level or pentaprism), dedicated lines of lenses (bayonet), slow speeds. The production figures cited here come from KMZ.
You can find details by year, on http://www.zenitcamera.com/catalog/cameraproduction.html.

Start: Production: 76503 between 1958 and 1964.
The Start has a shutter release button on the front and it's bayonet Helios 44 has a button to push this shutter button, while closing the diaphragm. Unfortunately, no other lens has been produced for this camera, the owner had to use the Zenit M39 range of the time, thanks to the supplied adapter ring. The advance mechanism of the first models was fragile, modified after 1961. There existed prototypes Start2 and 3, with internally coupled diaphragm lenses, and a selenium lightmeter in the viewfinder for the 2. Never serial produced.
Zenit 4: 19740ex between 1964 and 1968.
Zenit 5: 11616ex between 1964 and 1968.
Zenit 6: 8930ex between 1964 and 1968.
They succeeded to the Start. Main difference: use a central shutter (B+ 1 s to 1/500s). Also equipped with a selenium lightmeter (in the body, not in the interchangeable viewfinder), the display is in the viewfinder, not separated as in the Zenit E.
Zenit 5 has a motor incorporated in the base (the first 35mm reflex camera with built-in motor?) Zenit 6 is identical to 4, but delivered with the Rubin 1ts 2.8/37-80mm Zoom, copy of the Voigtländer Zoomar 36-82mm.
Apart from the standard lens Vega 3 2.8/52mm and Rubin zoom, again a very small choice of lenses (a 37mm, a 85mm, a 135mm, and another 50mm and 200mm never produced in series)