At some point likely in the late-'80s S&W got out of the grip-making business and farmed the work out to a number of smaller outfits. As you've found out, the "Smith and Wesson Grip Company" was one of them. Ace is another name you'll find on some factory grips, made for only a short time (S&WGC may have become Ace and then folded, I don't know). Eventually, they settled on Altamont, who continues to make them today.