Just an FYI for folks with magazines that have the old style followers: You'll want to get new style followers if you go with the "Extension Only" option.
I just tried using 2 of my old, non-drop free mags with "Extension Kit" (Yes, I'm weak and bought two of them so I'd have three 30 round mags) and the follower doesn't allow the spring to seat and it kicks over about half way through the extended mag body.
A few pics below for illustration.
Old Follower with the new extension spring which is the same as a standard new Glock spring:
Current Follower with new extension spring which is the same as a standard new Glock spring:
A group shot.
-Left Side: Current follower with current Glock spring.
-Center: Old follower with old Glock spring.
-Right Side: The magazine extension spring from KRISS.
At least new followers are cheap and easy to install.