EDC Plus 3 - Glock Magazine Extension Glock 9mm / 40 S&W by Henning Group
Henning Group's EDC Glock Magazine Extension fits all 9mm, 357 and 40 S&W double stack magazines. Extends capacity by 3 rounds in either 9mm or 40 S&W. (Fits Glock G17, G19, G19X, G22, G23, G24, G26, G27, G31, G32, G33, G34, G35, G45)
Extra power magazine spring is not required, but you have an option to add for extra spring pressure if you so desire. With a stronger spring, you may reduce the capacity by one round.