The 992 was Hamilton's first main 16 size, 21j railroad pocket watch. It was made from 1903 to 1921, in quite a few different variants. The very earliest ones were single roller with a fairly elaborate damaskeen pattern, composition jewel settings, and a brass center wheel. Later versions switched to double roller, a gold center wheel, gold jewel settings, and a simpler narrow stripe damaskeen pattern. There were also some "special" patterns, including two-tones and others.
One will sometimes also see "992P" and "992L". The P and L refer to pendant and lever setting, respectively-lever setting is by far and away the most common.
The 992E was a 992 fitted with a blue Elinvar(elastically invariable) hairspring and solid monometallic balance. The earliest of these had the same narrow-stripe damaskeen as the later 992, but most commonly seen is a wide-stripe damaskeen pattern. The pallet bridge has the word "Elinvar" stamped into it.
The 992B was a complete redesign with few if any parts interchangeable with earlier models. The key feature is the white Elinvar extra hairspring, although the plate layout and most of the other key visual features are different. The damaskeen is a narrow stripe, although all the plate lettering is filled in with black rather than gold as on earlier watches. The serial number generally starts with "C", although "4C" and "S" are also sometimes seen. As far as I know, all 992Bs will say "992B" on the movement. They are also often(although not always) fitted with dials that say "Hamilton Raliway Special."
There's a lot to learn about all three of these watches, and this answer is only intended to be an overview to help you spot and differentiate them. There's a lot of great information about all three contained in old threads. If you look at the "Hamilton threads" sticky at the top of the page, you can find some of these easily.