If you have the Glock G19 / G23 magazine, and you want to use it with your Glock G26 / G27 pistol, then this is the right sleeve to fill the gap. Will fit Glock G26 pistol with Glock G19 15 / 15+2 magazine.
ProMag Glock Magazine Spacer
A&G Glock Magazine Spacer Sleeve 21 to 30 Gl29-30 for sale online
glock 27 products for sale
X-GRIP Magazine Adapter for Glock 26 Compact GL26-27C
XGrip GLOCK 26-27C Magazine Adapter Magazine Spacer for Compact GLOCK 19/23/32 Magazines Installed Into GLOCK 26/27/33 Frames Matte Black Finish
X-GRIP Magazine Adapter for Glock G19, G23, G32 to G26, G27 and G33: MGW
XGrip GLOCK 26-27C Magazine Adapter Magazine Spacer for Compact GLOCK 19/23/32 Magazines Installed Into GLOCK 26/27/33 Frames Matte Black Finish
Koala (u/HelpfulWitness5447) - Reddit
Glock 19 In Pistol Parts for sale