I'll try to get a pic up of my '89 GP100 with the semi-permanent grip locator, if I can figure out how to do it with my new iPhone
The 2-piece grips look like the later one piece wood insert Large Letts, except they are a different, harder kind of rubber and are in two halves instead of a slip on grip body. They are all over Ebay and Gunbroker, so a search for "GP100 grips" and you'll likely see a set of the 2-piece Lett grips for sale on one of these sites. The GP100 in my earlier thread about my 1980's GP100 has the two-piece Lett grips, but it's hard to see a difference in the pic. I like the 2-piece Letts, they are "slicker" and not as grabby as the later one piece Letts, and feel great in my hand. There is a seam though, and they don't fit as clean as the one piece models.
The locator pin is fitted into the hole, in the earlier adjustable sight GP's and is not easily slipped in and out like the later and modern GP100's. It can be tapped out, but I chose not to do this with my 1989 GP, since I bought it to keep it original.
The early fixed sight GP100's do not have the semi-permanent grip locator and used the same Compact Lett from the start. Why Ruger chose to use a 2-piece grip on the adjustable sight GP's and not the fixed sight models is a mystery to me. I know fixed sight GP100 production is not thought to have started until late 1989 or possibly 1990, since Service and Speed Sixes were still being assembled from "cleanup" parts until as late as 1988 and some say even 1989. Adjustable sight GP100 production started in 1986, and Security Six production ceased in this year.