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)
Known serial numbers:
904 _______ A. Berry collection
2053 ______ Fed book (1939)
2404 ______ Communist cameras
5456 ______ seen on eBay
7555 ______ seen on eBay
8516 ______ seen on eBay
8789 ______ Leica shop
9263 ______ seen on eBay
9758 ______ seen on eBay
12358 _____ seen on eBay
13484 _____ seen on eBay
13594 _____ seen on eBay
14838 _____ PYC collection
100411 ____ PYC collection
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.