This is 8.5cm viewfinder made at KMZ (Krasnogorks) for 35mm rangefinder still image cameras. It allows you to see what image will fit inro the frame when you install a 85mm lens on to a camera.

But it seems that previous owner had a 135mm lens, so he modified this viewfinder a bit - he installed a matte to limit field of view. As you can see from the last picture, you can take the front plate off the body and remove the matte if you want to revert to 85mm field of view.

So, you have sort of 2-in-1 viewfinder for 85mm and 135mm lenses.