NON POUR BALLE - choked unrifled bores used 1878 - 1897
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

Note this gun was proved in 1930, and I do not know the post-1924 rules.