This is a STAINLESS STEEL 15/30-round magazine that holds .223/5.56mm caliber ammunition, developed by C Product Defense.
CPD used a process called Isonite, “ISONITE” is a high-performance variety of salt bath nitrocarburizing process. It is a thermochemical diffusion process whereby nitrogen-bearing salts generate a controlled release of nitrogen at the interface of a ferrous part. The benefits of this process include improved durability, corrosion resistance better than chrome plaating, lower material distortion, greater resistance to abrasion and seizure, heat resistance, and last but not least a beautiful black appearance.
These stainless steel 30-round magazines come from the factory with limiters that reduce capacity to 15 rounds. These limiters can be removed to restore full capacity or permanently installed. Magazines are triple-finished for superior strength and endurance, with dimensions that are optimized for feeding all types of rounds (9 including reloads).
Keeping a few extra factory magazines around is never a bad idea; extra magazines save loading time at the range and also serve as a backup for those days when you simply cannot remember where you put your primary magazine.