Beretta APX Compact Grip Frame, it's sold with three back straps: standard, small and large
Unique modular chassis system allows replacement, reconfiguration, size tailoring, including:
Different color grip frames
Customization of the frame without destroying the gun (stippling)
Compatible with Compact/Centurion uppers only. Not compatible with full-size upper.
APX Centurion (All calibers)
APX Compact (All calibers)