The first picture of the underside of the barrels appears to show a crolle pattern underneath the blueing, and the barrels do NOT carry smokeless proof. The gun was made 1898-before lettre annale marks starting in 1921; almost surely prior to WWI.
The 16.8 bore = .661" = standard 16g
The right barrel could have less than .008" constriction and not be marked as to mm.
The left choke is 16.3 = .642" = indeed Imp. Mod.
The action body was probably originally case colored, and the BTFE would be quite atypical for a Belgian gun of that era
The gun needs to be evaluated by a double gun specialist smith, and there are a number of excellent choices in Ill. and Wisconsin
If you'll click on 'Edit' you can delete the SKB images from your posts
