Part of the standard issue US MOLLE II system: Triple Magazine pocket 30 round. Aka, the triple M4 magazine pouch.
These are in OCP
NSN: 8465-01-641-9389
These feature three pockets, each of which will hold a single M4/M16 magazines. Retention is provided by a top flap with a metal snap closure. They have three MOLLE straps on the back, with additional MOLLE straps on the front.
These will be from a variety of makers, but all are genuine US Government issue and in excellent shape.