Gen3 17 & 19 both have Grip Force Adapters, a little polishing of the striker engagements, and either the Hogue or Pachmayr rubber grips. The Gen4 21 and the 19x have the large beavertail backstraps installed. The 21 has also had some polishing of the striker and a Pachmayr grip. The 19x has the Glock luminescent sights installed which work great.