A base pad adds weight at the bottom of the magazine so it exits cleanly on release and, where a magwell is fitted, closes the funnel geometry at the bottom of the grip for a more consistent incoming reload entry. The PX4 uses a rotating barrel design and has its own frame geometry and magazine, separate from both the 92 family and the APX. Its base pads are PX4-specific -- not interchangeable with other Beretta platforms.
Confirm compatibility on the individual product page before ordering.