I believe those grips were from the factory. One of three styles that I have identified for the TGT. Two custom walnut styles. One has finger grooves and the other doesn't. I believe the plastic are the least common and I suspect they must have had some kind of supply issue. I'd love to know when yours was made in relation to the end of production. I wonder if the supplier had a minimum order that would have exceeded the number of units S&W planned to build before they discontinued it?

There are also plastic woodgrain grips like the regular walnut factory grips. You can tell the difference in photos because the plastic ones do not have the gold medallion. It's embossed just like the black ones. I know some of those grips went to export guns.