Its could be a double sided magazine floorplate (the purple coloured object at the bottom of the pistol grip).

This allows a standard capacity magazine to be loaded, while allowing a second magazine to be available by unloading and flipping the magazine.

This is dumb for other reasons… Like dirt entering the exposed magazine, and causing jams/fouling after insertion.

Example floor plate:

https://www.opticsplanet.com/cross-armory-double-stack-magazine-floor-plate.html