As I understand it the "Standard" load for the British 2˝" shell was 1 1/8oz. During WWI the load was legally restricted to 1 1/16oz. Following the war the slightly lighter load was so well liked that many shooters stayed with it & si it more or less became a Standard. The guns themselves though as Larry pointed out continued to be proofed for the 1 1/8oz load. The 2 3/4" chambered guns were proofed for 1Ľoz.
It is noted that W W Greener stated at least as early as 1910, probably even earlier than that, that the 12ga would handle 1Ľoz shot so well that unless a heavier load was desired there was no advantage of going up to a 10ga. Sinjce I shoot round pellets out of a round bore I have never been overly impressed with a "Square Load".
