These are British style belt hooks that would be found on a typical Sea Service pistol.
There are two sizes. The 7" hook is what comes on our 1756 Pattern Sea Service Pistol with a 12" barrel, the 5" hook is for shorter barreled pistols.
They mount with the rear lock screw and keep from rotating via a pin that sticks out of the mounting foot. These can adapted to fit most pistols with wooden stocks. They are made of hand forged steel.