Rocketman;
To the best of my memory without going through my books the sizes between the whole numbered gauges came into use in 1887 at the same time they began putting the chamber mark on. Thus you will not find a 13/1, 12/1 etc on any British gun proofed prior to 1887 nor will it have a chamber mark. Since I did not see a chamber mark on this one I assume it to be a pre 1887 gun. Muzzleloaders had been gaged at the muzzles & some early cylinder bored breech loaders were also I believe. The general acceptance of choke boring forced the change to gaging at the breech.
The Not For Ball mark on early choke bored guns was due to it had been common practice to use closely fitting balls. In the choke bored barrels it was necessary to drop the size of a ball to one which would pass through the choke to prevent an obstructional situation. Many interpreted this to mean "NO" solid ball could be used but this was not the case. This mark was changed to the simple mark "Choke" to alleviate this thinking, but in many respects it came too late. Many still insisted that no solid ball could be safely fired through a choke bored barrel.
