This is the Check-Mate USGI 20 round 7.62x51 magazine for the M1A and M14 rifles.
Check-Mate Industries are the OEM manufacturer for Springfield and have been the primary supplier for the US military for over 30 years. They have an excellent reputation for reliability.
Precision manufactured from hardened steel, they have a milspec phosphate parkerised finish and heavy duty corrosion resistant springs.