So, I came across this pistol in the August 2014 issue of Muzzleblasts.
At first glance you would think both barrels fired at the same time.
Bu then you see this sliding pan cover exposing a second pan beneath the first, and a flash channel to the second barrel.
I thought this was a pretty brilliant solution for multiple shots. With a tight fitting pan cover, it seems pretty reliable for a fast second shot on a sidelock. I guess the second channel is drilled at a steep angle to enter the top right flat of the lower barrel. I may try to build a rifle/smoothbore with a set up like this one day. Have you folks seen any other guns like this?