Replacement hand stop. The edge your hand contacts has been radiused in, and no longer has the mounting channel going through it to improve comfort. It has been thicken by a small amount to allow the sling to go over the hand more smoothly. A recess is on the point where the sling mounts to allow a Larsen sling to sit flushly. The material is a slightly porous nylon, not glossy smooth, it feels very nice in hand.
Uses Altius hardware or my "Mounting Block" The original screw is to short with a Larsen sling, you will need an M6-1.00x20mm machine screw (availiable from most hardware stores).