That's an excellent find!
If memory serves right, the G200B was introduced in 1994. It is identical to the G200 with one important distinction: it changes the unbalanced poppet of the G200 to a balanced one, hence the "B" in the name.
This is somewhat analogous to how the C350 and C370 relate to each other.
The G200B uses a two-piece poppet, which isn't ideally shaped from a gas flow point of view. The poppet also lacks a wiper O-ring on the balance chamber end. This poppet should be inferior to a modern one-piece design, such as the S-wing poppet.
And here comes the interesting bit: the G200B is well capable of running the updated S-wing poppet, which was introduced around 1999 for several ScubaPro second stages. But which poppet you have in there is anyone's guess until they open it up.
I also enjoyed how this design utilized the plug that envelopes the balance chamber. It is the analogues to the micro-adjust, but it also means there's no diver-adjustable knob (something unneeded in my humble opinion anyway...). The plug allows for variable spring tension, meaning that ScubaPro was able to be less picky about spring quality, which was a major headache back then (and still is...)
I also liked the screws and physical clips instead of the modern bars and "click" connections. The screws should be imperial 3/32" hex key if I'm not mistaken.
That corrosion you have on there is virtually nothing. I'm genuinely surprised to see so little.
In short, this is a great second stage. It is very similar to the revered G250, minus the diver-adjustable knob. In fact, they share many of the parts, including the actual housing. It's a keeper that you should enjoy, especially when updated to the S-wing poppet.
