This is the CPD Duramag AR-15 6.8 SPC 10-round stainless steel magazine.

CPD® re-designed the 6.8 SPC magazine from the ground up and the results have been astounding.  Due to its specific geometry combined with CPD®’s proprietary Teflon coating, this stainless steel magazine continues to keep firing on all cylinders with the dirtiest of ammunition.