These units replace the magazine base plate and internal floor plate. They will add two rounds to the Glock M26, one round to the Glock M27 and M33 and no rounds to the Glock M39. These units will add 5/8" additional length for better control and comfort.
Does not add extra round to either Glock 26 or Glock 27 mags but I do like that you have somewhere to put your pinky finger. I guess I will buy more.