This is the 33 round Glock factory 9mm Gen 4 magazine for the Glock 17, 19, 26 and 34 pistols.
The Gen 4 type is backwards compatible with all previous generation Glock pistols, as well as the newer Gen 5 (with the magazine catch installed in the default position).
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.