The controllers at the
Banc D'Epreuves Des Armes a Feu De Liege seemed to enjoy messing with gun for U.S. export
The usual c. 1900 Belgian 12 gauge bore would be 18.4 mm = .724"
Conversion table: 1 millimeter = 0.039 inches
http://www.convertunits.com/from/mm/to/inchesMARKINGS
1878 - 1897 - NON POUR BALLE = choked (at least .2 mm = .008") unrifled bores.
1898 - 1910 – Bore in mm (22 cm from breech) and muzzle (choke constriction) appear
next to each other after ‘choke’
1910 – 1924 – Bore in mm is
over muzzle dimension
For instance: a 12g barrel with 18.3 & 17.8 has a .720" bore & a choke constriction of .700 or .020” (Modified).
If marked only with one number & the word ‘Choke’, then it indicates bore size only and has at least .2mm (.008") of choke.