Hi Larry,
Thanks for sharing that video. It is an exceptional gun by the greatest flintlock gun maker ever. As I understand from reading the actual patent for Manton's gravitating stops, their purpose had nothing to do with preventing the shooter shooting vertically. In fact, having hunted birds a lot with a muzzleloading shotgun, I have no qualms shooting near vertically if it is a proper shot at the moment. The reason for the stops was to prevent an accidental discharge during loading when the barrels are vertical. With a double barrel gun, you are often loading a barrel after the other is loaded so you are over the muzzle of a loaded gun. The gravitating stop was a back up to half cock. In that capacity, it replaced the sliding safety bolt, which you do not see on the Manton example because it is replaced by the automatic safety action of the stops. Many hunters removed the stops because they interfered with shooting at an extreme near vertical angle. They also had to be kept clean to work properly.
dave