IMI Defense Single Magazine Pouch for Glock Pistol. Since all Glock magazines are of the same width, it can fit a HUGE variety of them ranging from the G17 to the G36.
If you remove the mag pouch itself, you can apply it to these holsters:
IMI-Z1370-TN IMI-Z1370
Fits the specified magazines:
G17/19/22/23/26/27/31/32/33/34/35/37/38/39 PX4 H&K P30