This is a Magpul Industries Accessory known as the MIAD/MOE Storage Core, model MAG057-BLK.
Internal core unit for the MIAD as well as the MOE(R) and MOE-K2(R) Grips* designed to hold one (1) standard AR15/M4 complete bolt and firing pin.
Features:
Designed to hold one (1) standard AR15/M4 complete bolt and firing pin