This extension replaces the magazine floor plate adding two rounds capacity to Glock factory M20,M21,M29,M40 and M41 magazines. This unit is for use on high capacity magazines only, it will not work on the 10 round M20 and M21 magazines. Not for use on Glock M30 10 round magazines. These units will work on both third and fourth generation Glock models These units will work on Glock M30 9 round magazines, enabling the magazine to hold 11 rounds.