It was made 1910-1920; I do not see a lettre annale; likely pre-WWI
It does not carry Smokeless Powder proof but Belgian guns for export frequently did not
The 16g bore is 16.8 mm = .661" with muzzle 16.5 mm = .650" = .011" or 16g Modified

Unfortunately, neither Pete Mikalajunas' list, nor http://www.hlebooks.com/daubres/li02.htm have a Belgian maker that I can make into 'FBA?'