Never seen or heard of this one before. It is a 20 rounder (thanx dpmmn). Markings on floorplate are as follows.
(to the left) S in a box
M16/M16A1 CAL 5.56 MM
Universal Inds. Div.
Simmonds Prec. Prod. Inc.
West Haven, Conn. USA
Just wonderin if anyone had seen this before.
yes on older 20 round magazines