This is a SB Tactical Stabilizing Brace known as the TAC14, model 870-SBM4-01-SB.
This SB Tactical SBM4 Stabilizing Brace Kit for the Remington 870 TAC-14 is ready to assemble on your Remington 870 TAC-14. It will greatly enhance the usability of the firearm by adding a third point of contact for stabilization. Kit includes: SBM4 Pistol Stabilizing Brace" STDT-C pistol receiver extension" ERGO Flat Top SUREGRIP" ERGO Remington 870 adapter" castle nut and adjustable nylon strap (1.60" wide).
Features:
SBM4 Stabilizing Brace
Complete kit, ready to assemble
Fits Remington 870 TAC-14 12 gauge