You can't know without the hang-tag. Some 12-gauge guns show 1 1/8-ounce --
107705 01 12-gauge, 2 3-4 inch, 1 1-8 ounce.jpg
and some 1 1/4-ounce --
108998 12-gauge, 2 3-4 inch, 1 1-4 ounce.jpeg
Then some are totally confusing. This tag for a 1925 vintage gun shows 1 1/4-ounce and 2 5/8-inch shells?!?
213855 12-gauge, 2 5-8-inch, 1 1-4 ounce.jpg
Our U.S. ammunition manufacturers quit offering 1 1/4-ounce loads in the 2 5/8-inch 12-gauge shell between 1906 and 1907. From then on, you had to go to the 2 3/4-inch or longer shells for 1 1/4-ounce.