Chris's camera pages
Stereo attachment for Kodak Retina IIc, & IIIc cameras
The somewhat rare accessory shown here on a Retina IIIc camera is a prism-type splitter, and produces a stereo pair side-by-side on a single frame.
If the viewfinder mask frame is removed from the stereo attachment it is also possible to use it on the original model Retina Reflex too. There shouldn't be any problem using it on a Retina Ib either, but you should note that the viewfinder mask will not be in a useful position, because the Retina Ib finder is in the centre of the camera, not to one side like on the IIc and IIIc models.
At one time Kodak had a mounting service available. If my memory serves me well, there may have been a box to tick on the processing envelope to alert the lab that the film was to be mounted in the special stereo frames. The cardboard mounts had a narrow bar down the centre to cover the area where the images met.
Naturally, Kodak also made a viewer to suit.