The term "hook-breech" means that the breechplugs screwed into the breech end of the barrels are angled or slightly hook shaped; instead of being held to the wooden stock with screws the hooks fit into the holes of a separate standing breech, which is held to the stock by screws. It is a method of making the barrels removable without the use of tools. One simply drives out the wedge, which is a metal 'key' which goes in one side of the forend, through the wedge tennon (the small loop on the underside of the barrel, shown in your last set of photographs) and through the other side of the forend. Then the muzzle is lifted upwards and the hooks 'unhook' from the standing breech and the barrels are completely detatched from the stock. If the wedge gets loose and falls out unnoticed, the barrels will detatch themselves as soon as the ramrod is removed. If they hit the ground nipple first they sometimes go off, and were often lost overboard from boats.