Sir Gerald Burrard discuses this at length in his "Modern Shotgun". The shot wt was a mark found primarily on guns proofed in England. 1 1/8oz was for 2½" chambered guns & 1¼oz for 2 3/4" chambered guns which carried a higher pressure than the 2½" guns. Shells were loaded in both lengths with hevier shot loads specifically for the guns they were to be used in, as the 1¼oz "Low Velocity" load in 2½" shells. This caused much confusion among users as many assumed the gun was not proofed for the shells. This was the primary reason for changing to the "Tons" mark rather than shot weight. Actually each shot wt proof had a specific pressure it was proofed to, which was the determing factor, rather than actual shot wt.
