This is a BLACKHAWK Magazine Pouch, model 52BS17BK.
Constructed of lightweight nylon ripstop" this magazine pouch is designed to fit most collapsible stock designs. It has an adjustable lid to accommodate 20- or 30-round magazines.
Features:
M4 Collapsible stock magazine pouch
Adjustable lid for 20- or 30-round magazines
Dimensions: 3L x 4.125W x 1D