I think I figured out my GP100 grips. The grips I took off my current GP, a 6 inch SS, are one piece. The two piece grips I have in the gun safe are from my 4inch GP100 which I had in 1990. I think the two piece are for a round butt GP100, (typically a three inch.) I like the Hogue grips so much I may try to sell the other grips as a pair.
I understand what you are saying about a 3" GP having a different grip. However I believe that it is only the grip, and not the frame, that is different (in contrast to a "round butt" K-frame S&W). In other words, a 3" GP comes with a "compact" grip, but it will still accept the standard factory grip, either the discontinued style or the newer Hogue style. Likewise, you can put a compact grip on a 4" or 6" GP (or a Super Redhawk, for that matter).
Two piece compact grip? That I don't know about. The grip on my 1988 vintage 3" GP is just like the standard grip (one-piece construction with wood inserts), but smaller.
Don