Shortly before WW-II, the manufacturers began putting up 28-gauge Skeet Loads in 2 3/4 inch cases and it wasn't long after the war that the 28-gauge was standardized at 2 3/4 inch. Western Cartridge Co. offered the 1 ounce 28-gauge 2 3/4 inch Magnum shell from 1961 to 1967 --
Super X MAGNUM 28-gauge 1961 to 1967.jpg
which Jack O'Connor wrote quite a bit about. I'm not sure when they reintroduced the 1 ounce load as my ammo catalog collection gets sparse after 1970.
Winchester-Western 1 ounce 28-gauge shells.jpg
Winchester 1 ounce 28-gauge shells.jpg