The finder allows to compose picture while holding the camera perpendicularly. A pivoted mirrored window (see rear view) makes it possible to use a rangefinder. I believe there was a Leica accessory of this design, and it was copied by the Russians.
(Source www.fedka.com)
90 Degree FED Finder
his is an unusual and rare 90 degree finder for the FED cameras. It was made in the 1930's by the FED commune.