The package containing a 12 round mag for the 365xl includes a base plate extender. What is this used for? Thanks, Dennis

It's for the 15-rounder.

Compactsig, I believe the additional base plate part is for using the 12 round magazine in a P365. The 12 round mag is flush with the grip of the P365XL, but would extend out of a standard P365.

Yeah the 12rd mags and the 15rd mags both come with 2 base plates. One is for the P365xl and one is for standard P365