As I recall during the period of time the British proof houses stamped the barrels with the shot load proofed for they were as follows in 12 gauge;
2.5" - 1 1/8oz; 2 3/4" = 1oz & 3" = 1oz.
The actual shells were loaded with shot weights both heavier & lighter warranted for use in a gun having proper chamber length of the shell.


Miller/TN
I Didn't Say Everything I Said, Yogi Berra