Picked up an early CAR sliding stock detent plate at a show. This has little finish left on it, but has an extra hole on it on the left side, as seen from the stock, like for a snap ring. Is this an early XM-177 detent plate set up for a side sling, or an aftermarket piece? Got this from a parts dealer who always has the cool stuff, early AR barrels, grips, stocks, etc. He gets a lot of his parts from a friend of his who spends his time traveling around picking up cool stuff from a bunch of places. Any info appreciated. Thanks.