This is a Soviet made Zeiss type Universal Finder , a well constructed and convenient accessory for your rangefinder. This accessory is one of the best items made by the Soviet photo industry.

* Supports the following focal lengths: 28, 35, 50, 85 and 135 mm.
* Parallax correcting marks on the turret dial (down to 1 m) and a crosshair on the image screen.
* Allows to see the objects beyond the selected frame, which helps to compose the picture (the early models only showed the frame).
* This finder is commonly used with the Soviet Leica and Contax copies, but can be used with any rangefinder - it mounts in the accessory shoe.

(Source www.fedka.com)