I am not going to write a lot about this because it's been done better elsewhere. Arguably the best site is probably DrT's Stereo Realist page.
There are three major "Realist" models. #1041 was the first and most common (and least expensive), which has 3-element ƒ3.5 Ilex lenses; the #1042, which has 4-element ƒ2.8 Ektar glass, and the #1050, the "Realist Custom," which also has ƒ2.8 lenses. Most people have either the 1041 or 1042 models, and the only significant difference is the optics.
DrT's site has some info on aging based on serial numbers. My SN is A85330, which puts it between 1954 and 1956. Maybe.
Mine also has a little plaque on the foldover lencap that says "Realist StereOO Retored by Zak." Thanks. Hopefully it'll come off with a little bit of persuation from a hair dryer and my fingernail.
The Realist wasn't the first stereo camera by a long shot, but it was the most successful and probably the longest lived.
Camera manual: Orphan Cameras.com