Rubberhead,
What you have in the toe of your Browning's stock is a cartridge trap. Bullets are the projectile that are propelled out of the barrel. A cartridge is the case, primer,powder & projectile (bullet) that make up the cartridge that is loaded into the breech of a firearm prior to firing.
I've only seen a few of the Browning/FN Express rifles & none w/a cartridge trap in the butt stock but I'm not a Browning collector so who knows what was available if you were willing to pay for it.
I tend to agree w/graybeardmm3 that that the cartridge trap is not original. I remember an aftermarket cartridge trap for the toe of the buttstock similar to the one you have pictured being sold some years ago but I can't remember where.
It looks like a nice installation regardless of who did it
