This is the hinged magazine base for the Browning BAR 3, Browning BAR 4X and Winchester SXR2 in short action sizes.
The steel box magazine assembly (available separately) clips into this floorplate.
If you want spare magazines ready to go, you will need this floorplate as well as the magazine, as in practice clipping and unclipping mags into the floorplate in the field is fiddly and really needs a tool.
Comes with a Browning logo sticker on the base (removable).