I was lucky with this one. I happened to notice it on the shelf within the office of a local auction house. The owner let me have it for 2.00 (GBP)! Keystone are an American manufacturer from the turn of the Century (maybe a little later) who mass produced viewers which were widely used at the time for educational purposes in schools. It is of a classic Holmes style design upon which so many viewers were based. The viewer takes cards measuring 3.5" x 7" - the kind that can often be found at flea markets and car boot sales (if you are lucky!). The resulting stereo picture and the ease of adjustment/focus via the slidebar is enough to make many standardise on this format as their format of choice (at least for prints)/
(Sorry about the picture, a better one will follow)