This is a turret finder for rangefinder cameras. It is used by attaching it to the accessory shoe on the top plate of the camera, as shown in the product photos. (The Leica M3 in the photo is not included with this product) By rotating the disc-shaped turret with the objective lenses, the finder field of view switches between: 28mm → 35mm → 85mm → 135mm → 50mm (→ 28mm) The exterior is a bit worn, but the finder image is clear. The turret clicks firmly into place. Cameras, lenses, and accessories made in the USSR (Soviet Union) during this era were often well-made mechanical products, a world of difference from modern Russian products. This turret finder also feels precise in its mechanical processing, in addition to its excellent optics. It was useful for using wide-angle and telephoto lenses with rangefinders, including Leica. I obtained it at a camera store in London, where I used to live, and have stored it in a dry cabinet. It comes with a dedicated case that fits perfectly.