Find it here - https://doublemdefense.com/product/magpul-mag661-blk-pmag-21-gl9-9mm-glk/

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UPC: 840815109709
SKU: MAG661
Item Description:
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.
Item Specifications:
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 floorplate 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