I expect that the 28-gauge "magnum" was 3-inch.
No, it was/is a 1960 follow on to the 2 3/4 inch Magnum 12-gauge (1 1/2 ounce), 16-Gauge (1 1/4 ounce) and 20-gauge (1 1/8 ounce) loads introduced in 1954, put up in the, since WW-II, standard 2 3/4 inch 28-gauge case.
Earliest ones the paper cased shells in the box upper left.
For the first thirty years of the Twentieth Century our North American 28-gauge ammo was either 2 1/2 inch with 1 3/4 drams of bulk smokeless powder pushing 5/8 ounce of shot --
or 2 7/8 inch with 2 drams of bulk smokeless powder pushing 5/8 ounce of shot. I've yet to ever find a box of these.
Then along about 1931/2 Western Cartridge Co. added a 28-gauge load to their high velocity, progressive burning powder, Super-X offerings of 2 1/4 drams equiv. pushing 3/4 ounce of shot out of the 2 7/8 inch case.
During the 1940s the 2 3/4 inch 28-gauge case was introduced and by the time WW-II was behind us the 2 1/2 and 2 7/8 inch 28-gauge cases were gone. In their January 3, 1961, catalog, Western Cartridge Co. added the 28-gauge 2 3/4 inch Magnum with 1 ounce of shot to their offerings.
