This stereo lens and the viewfinder mask is a counterfeit piece, probably originally made in Poland (maybe the last version in Russia or Ukraine). Probably made with lenses from a Smena and some Zorki casing parts.
It is a copy of the Leitz Stemar stereo attachment for the Leica, however it is incomplete since they never come with a prism attachment to fit over the lens and make the distance between the two lenses greater for full stereo effect.
There seem to be three versions which are easy to tell apart by the details of the engraving. These three versions are shown in the photos and known examples are listed under the version discription. Princelle states that there were probably less than 50 examples made of the first version and probably the other versions are rare as well.
1st VERSION: Chrome front plate with unpainted engraving.
No. 44 - Princelle 2nd Edition (page 109)
No. 47 - Collection of Bill Parkinson
No. 82 - On Ebay, March 2010
No. 88 - Arsenal Photo, Germany
2nd VERSION: Chrome front plate with engraving filled with black paint.
No. 027 - Viktor Suglub Museum Collection
No. 030 - Shown on Novacon.com & Molotok.comNo.
No. 205 - Collection of Alexander K
3rd VERSION: Lens casing and front plate painted black with engraving filled with white and orange-red paint.
No. 019 - Shown on Novacon.com