This is the 33 round Glock factory 9mm Gen 5 magazine for the Glock 17, 19, 26 and 34 pistols.
The Gen 5 type is backwards compatible with Gen 4 Glock pistols, also a popular choice with pistol calibre carbine rifles such as the Schmeisser AR15-9 SP9.
Manufactured from hardened steel incased in a tough polymer outer these are very durable.