Barrel extension into standing breech, with Greener-type cross- bolt, and sideclips, if you please! That's a lot of class for a boxlock, which is why I think Francotte, or maybe Dumoulin. Seems too well engineered to simply be a Belgian "trade gun."
In fact it is a quite common package for an old school shotgun from Liege. The same with my "bayard" that was known as a basic non custom / non fancy gun. Belgian's gunsmithing good old and lost time .
. 
