Magpul PMAG 21 GL9.

The PMAG 21 GL9 is a 21-round Glock 9mm handgun magazine featuring a new proprietary all-polymer construction for flawless reliability and durability over thousands of rounds. Meeting the overall length requirements for a 140MM competition magazine, the PMAG 21 GL9 offers additional capacity without the need for expensive extensions. High visibility controlled-tilt follower, stainless steel spring, easily removable floorplate for cleaning, paint pen dot matrix for mag marking, ridged floorplate edges for better grip, and capacity indicator windows. Drops free loaded or unloaded. All with the same boring reliability you expect from an OEM magazine.

The PMAG 21 GL9 is compatible with all full-size, compact, and sub-compact double-stack Glock 9mm variants with some protrusion below the grip.
FEATURES

Compatible with all full-size, compact, and sub-compact 9mm double-stack Glock handguns
Durable, lightweight, and extremely stiff polymer construction
High visibility controlled-tilt follower
Long life stainless steel spring for corrosion resistance
Flared, easily removable floor plate
Dot matrix panel for magazine marking and identification
Magazine indicator windows on both sides of body at 10, 15, and 21 rounds remaining
Drops free loaded or unloaded