It's a handguard ferrule.
It's the piece that usually has to be removed from the original wood handguard and installed on the new aftermarket handguard.
It's often butchered by drilling, grinding, and punching.
Even if it isn't butchered, it would have to be swapped back onto the wood handguard to put the rifle back in original condition.
All the online sources for them have dried up and they've become rare as hen's teeth.