Your pic is not big enough on the bbl. markings. I can make out the Belgian 'EL' but that's all. Can you do a couple of macro shots, one on each bbl,?. I think that would help. There are several folks here w/good knowledge & ref materials, but they will need to see the markings as clearly as possible to better assist you. You might also do a good lead pencil rubbing on a sheet of white paper and then take a pic of that too.
I'd sure like to see a better photograph of those hammers. There could be a clue to the maker there. I would also contact the Proof house in Liege for their recommendations or thots about who to contact for further information on a muzzle loader like you have and also contact the arms museum there in Belgium and see if they may be able to assist you. A good clear set of photographs of the hammers, locks & any marks in addition to good clean photographs of the gun will be of great aid to the curator there. In any correspondence convert the measurements to millimeters & centimeters and the weights to kilos yourself. They don't always do good conversions abroad.
Here, there is a publication called Arms & the Man, or something very close, that deals with many antique arms and they may be able to help you with identification. I'm not a collector so do not subscribe to it and I may have the title wrong too, but someone here will jump in and correct me if so.
The 'Fine London Twist' in bold gold on the rib could have been but a marketing ploy as a lot of games were being played in those days. I wish we had Oscar and Russ around to inject their thoughts. I don't pretend to know, but I really like those hammers!
Hope something here may help you in some small way.
