This is a Magpul Industries Adapter known as the MS1, model MAG519-COY.
Combine with the MS1 Sling and QD Sling Swivel to provide two to one-point convertibility. Melonite(TM) and manganese phosphate finished steel with custom snag-free high durability 1-1/4" webbing. Made in U.S.A.
Features:
Heavy-duty push-button QD Sling Swivel for one-point attachment
Light weight yet durable hardware
Provides weapon quick-release capabilities
