I have an opportunity to purchase an unfired older S&W Model 15 with 51/2 barrel from a friend. He got if from his Dad who recently deceased. The gun has a modification that neither of us can figure out. In the frame right behind the trigger is a sliver of silver of metal. The metal sits in a cutout in the frame and projects out maybe an 1/8 to a 1/4 of an inch. I think it’s a trigger stop. But I have never seen one mounted on the frame. It looks like a factory job. Does anyone know what it is?