I believe the gun was manufactured by Joseph Cap
http://www.littlegun.be/arme%20belge/artisans%20identifies%20c/a%20cap%20joseph%20gb.htm
between 1910 and 1921 or so (in the absence of a lettre annale)
It is Nitro proved, and has 70 mm = 2 3/4" chambers
The original bore was 18.3 = .720"
The right barrel was choked 17.6 = .693" = IM
The left barrel was choked 17.3 = .681" = Full