From the introduction of the .410-bore/12 mm/36-gauge (sic) shotshell in North America circa 1914-15, the 2 inch shell carried a 3/10 ounce shot charge and the 2 1/2 inch shell carried a 3/8 ounce shot charge.



The 3-inch shell with 3/4 ounce of shot was introduced with the Winchester Model 42 in 1933 --


July 1933, National Sportsman

By 1934, the 2 1/2 inch shell with the half ounce of shot we know today was becoming available.