This is a Magpul Industries Grip known as the MOE Grip, model MAG415-FDE.
The MOE Grip (Magpul Original Equipment) is a drop-in upgrade for the standard AR15/M4 pistol grip. The ergonomic" hand filling design combines anti-slip texturing with storage core capability. With a similar shape to a 'medium' sized MIAD" the one-piece reinforced polymer construction provides simplicity and a reduced cost while still maintaining the durability needed to withstand operational environments. The MOE Grip accepts optional Storage Cores for gear stowage and includes a basic grip cap. All mounting hardware included.
Features:
One-piece reinforced polymer construction
Beavertail backstrap for enhanced ergonomics
Positive anti-slip texture
Accepts optional Storage Cores