I believe that the majority of British "2" chambers actually measured from 2 9/16 to 2 5/8". The 1 1/8 oz proof was a nominal 3-ton proof. A gun proofed for 1 oz was proofed for 3 tons. I seem to recall someone mentioning here earlier that Greener, & perhaps others as well, had some nominal 2" chambered given a 1 oz proof.
According to Burrard the British also loaded a line of "Low Velocity" shells which used a heavier shot charge at lower velocity for a given cartridge length. The 12 gauge 2" low-velocity load was 2 3/4 DE-1 oz. This was I understand a predominant reason for the change from the shot weight proof to the Ton mark. This 2 3/4-1 load met the 3-Ton proof but many felt it did not meet the 1 1/8 oz proof.
Similar reason for the change from "Not For Ball" to "Choke". The choked barrel could be safely fired with Ball, as long as the ball was undersized enough to pass through the choke.
