This is a Primary Weapons Systems Stock known as the Enhanced Buffer Tube, model 4G2BTMB1.
PWS Enhanced Buffer Tube with Ratchet Lock Castlenut and Endplate is an enhanced AR15 Buffer Tube Gen 2. It is extruded from 7075 aluminum. It has a mil-spec tube diameter" extended carrier support lip for smoother cycling and six positions for stock lightening cuts.
Features:
Mil-Spec diameter
Anti-Tile Lip
Six Position with enhanced drainage ports
QD sling swivel mount end plate
Ambidextrous Push Button Sling Points