This is a nice and uncommon accessory for the Zenit SLR cameras - a stereo attachment kit, model SKF-1.
The kit includes two main parts - the stereo attachment that mounts on the filter ring of a lens and the stereoscope.
The kit is quite complex, overengineered, but fully usable. The beam splitter prism places two images on one 35mm frame. The stereoscope attaches to the beam splitter which also serves as a film holder.
The unit is intended to work with the Helios-44 lenses, or any other lenses with a 49mm filter thread.
Please download attached Luiz Paracampo's article on how to work with SKF1
See also the Chinese stereo projector at http://www.ussrphoto.com/Wiki/default.asp?ContentID=444&ParentID=2&WikiCatID=55 useful for SKF-1 Zorki stereo attachment and Kiev stereo atachment