Try this link;
http://www.gunproof.com/Proofing/proofing.html
This is from the Birmingham Proof House & you can scroll down the page & find the marks for 1925, 1954 & 1989. 1904 through 1924 are same as 1925, except length of chamber is not marked. 2½" guns were marked 1 1/8oz & 2 3/4" guns were marked 1¼ oz. 3" or longer were marked with an LC under gauge in the diamond to indicate "Long Chamber".
Incidently the smaller the bore the higher the pressure walls of equal thickness will contain, or for the same pressure can have thinner walls. It is however, as mentioned, easier to load the 12ga to lower pressures than smaller bores, unless exceptionally light charges of shot are used. Many will load 1oz in the 12ga, but not many go down to 13/16oz in the 16ga which would be an equivelent load.

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