The nozzles are Sykes patent. Also known as Irish pattern.

As they would hang nozzle down from the shoulder I suspect that they would leak if used with powder on one side.

I think that both sides were used with shot, either different sizes or to load both barrels together to avoid double charging one barrel of a side by side.

Used in British Isles until people gave up using muzzle loaders, and no doubt in the Empire and Colonies.

Any markings?