The proof marks are Belgian and date it between 1877 and 1893. 16.6 is the bore diameter in millimeters the other letter stamps could be maker's marks or workers assembly marks. I'll dig through my data and report back what I find if I find anything.
Grizzley1 is right, but I think this gun is older and dates back to the first half of the XIXe century.
. For sure, it's a Belgian shotgun from Liège (ELG stamp, but I cannot count the number of branchs of the star)
The barrels have the "provisionnal proof" ,E and L in english letters (near the 16,6 marking referring to the diametre in mm). The "perron" (the Column, representing a monument in Liege) is the proof for the mechanism. The letter with a star on it is the controller.
The diametre being 16,6 millimeters, it is a 16 gauge (16,8mm) with a choke.
JM could be Joseph Monisse but this does not really match with the time frame of production of this shotgun, as he was active much later in the century
I think this marks are the ones of sub contractors.
The style of engraving is familiar so I will search a little bit more.
Is there any marking on the wood stock? This stock was of high quality.
Is there any trace of markings or stamps above the barrel, in the 1/3 first part of it?
On the back of the trigger guard, it is not clear if it is just an engraving or a kind of signature.
Thanks, I first thought this could be for Simonis Nicolas, a gunsmith from the province of Liege , as the period matches but his stamps were NS and not SN.
will ask one person in Belgium who is an expert.
I have received an answer from Alain Daubresse, who is a specialist on Belgian guns and who wrote some reference books on this issue (but in French).He corroborates that these small letters markings are the ones of sub-contractors. The bars are identification signs of "equipeurs" (a kind of crewman) in charge of assembling guns.
I am afraid it will be difficult to go beyond that with the known elements and without any other traceable hints.