This is an HK® MP5 9mm 30-round magazine from KCI®.
The Heckler & Koch is one of the most iconic, most recognizable and most successful submachine guns in history. It’s also surprisingly expensive—but the magazines don’t have to be. These 30-round steel magazines from KCI were made in South Korea and designed for use with H&K MP5-pattern firearms including the MP5k, SP89, SP5K Zenith, MKE, Omega, POF, HK Contract MP5 variants, SW5, BW5, CA94, D54, V94, clones, and other custom builds.