This is a Magpul Industries Magazine Accessory known as the L-Plate chambered in 223 Remington, model MAG024-BLK.
The Magpul L-Plate is a floorplate replacement for current issue aluminum USGI 30-round magazines that incorporates a rubberized pad. Bumper pad is made of durable Santoprene rubber overmolded onto a stainless steel plate.
Features:
Floorplate replacement for current issue aluminum USGI 30RD mags
Provided improved speed and controllability during tactical mag changes