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.
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