Henning Group's Blueline Glock 19/23 Magazine Extension increases capacity by 3 rounds. Comes with extra power magazine spring.

Works for both the Glock 19 (9mm) and Glock 23 (40 S&W) - Compact length magazines. Extends the capacity as well as the grip. The "hook" extension allows for the support hand to utilize the extension for improved recoil management.
If you have a 19X you want to get the Glock 17 magazine kits instead.

Designed in collaboration with a current SWAT Operator and US Army Veteran, our Magazine Extensions offer:           

Confident magazine indexing through our patent pending design.
Improved grip support for shooters with larger hands.
Basepad allows for an effortless strip with the knife-edge of the support hand on malfunctions or stuck magazines.
Designed by a door-kicker, for door-kickers.

The kit comes with magazine extension and a longer magazine spring.
U.S. Patent 9,772,151