Universal viewfinder

This tool is essential for the Zorki user, and also for some Leica users. Before the generalization of zoom lenses, the SLR user could also use it to choose what lens he would need.
That is to say that when you could find one in a shop, you had to take it.
On a Zorki, or Fed, or other RF camera, the accessory shoe (in that case you don't say flash shoe!) is placed at the vertical of the lens, to avoid horizontal parallax error. For the vertical parallax error, you can turn the turret by a little for an approximative correction for close-ups (distance indications on the side).
Already existed under the name "Carl Zeiss Jena", but "mirror-reversed" compared to this one (turret on the left of the holder); the first Russian finders were also like that, it seems that they have been changed further for a practical reason (access to the shutter-speed knob)