The shotgun is definately Belgium made. Hope the below helps. Unforturnately I could not located any marks to indentify the maker altough the crispness of the stamping would indicate early or mid 19XX.
From a post in Shotgun world:
Proofs 1903 to 1924
Shown are the typical Belgian proofs on a 16 gauge A5 of 1910/11:
1 = Inspector's mark which could be any letter of the alphabet. The letter is depending on the time frame when the gun was proofed. E. g. the 1903 proofed A5s have the inspector’s mark “star/J”, while the A5s proofed in 1904 and later will have “star/T”.
2 = Smokeless powder proof (P.V. = Poudre vive = smokeless powderr)
3 = the Perron; indicates inspection or proof of breeching system
4 = Nominal gauge designation applied to shotguns given black powder proof (C = calibre; 16 16 gauge, 12 = 12 gauge and so on)
5 = Introduced to designate arms proved in conformity of German Proof Law of 1891. In now represents definitive black powder proof except for muzzle loaders.
6 = Numerator: Chamber dimensions in millimetres shotguns undergoing smokeless proof. Denominator: Length of chamber over diameter of chamber. In the example given the chamber length is 65 millimetres ( = 2 9/16”) and the chamber diameter is 18.7 millimetres. An other typical chamber length (on 12 gauge A5s9 is 70mm = 2 ¾”.
7 = weight of the barrel in grams when proofed. In the example: P. (= Poids = Weight) 0 (=zero) K. (= kilograms) 827.8 grams ( 1.8 pounds).
8 = Choke
9 = provisional proof of barrels