This is a SB Tactical Stabilizing Brace known as the SBM4, model 590-SBM4-01-SB.
This SB Tactical Stabilizing Brace Kit for the Mossberg 590 is ready to assemble on your Mossberg 590 Shockwave. 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 Mossberg 590 adapter" castle nut and adjustable nylon strap.
Features:
SBM4 Stabilizing Brace
Complete kit, ready to assemble
Fits Mossberg 590 both 12 and 20 gauge