This is the ASC ASC AR-10, SR-25 .308/7.62x51mm 5-round steel magazine.

ASC .308 Winchester magazines are engineered with stainless steel and quality internal components.  

Manufactured 100% start to finish in the USA, using .020 gauge 410 Stainless Steel Alloy, heat treated for superior strength and lasting endurance.  

ASC magazines are also chemically treated for corrosion resistance and coated with their proprietary finish, Marlube, which is a wear-in coating that burnishes into the surface providing self-lubricating properties.