This is a ACE Folding Stock Mechanism known as the ACE, model A500-K.
DBST-ACE's Standard Folding Mechanism (FSM) with Boss" is the standard Folding Stock Mechanism. It fits between the Receiver Block and Modular Stock. The FSM can be mounted to fold either left or right. It will add .75 to your LOP and weighs 3.5 oz. The Boss is the raised (oblong shaped) portion on the Arm (small side).
Features:
Standard folding mechanism with BOSS
Can be mounted to fold either left or right
