The original 12-gauge 3-inch Super-X load of 1 3/8 ounce of shot was introduced circa 1924/5. Western Cartridge Co. introduced their Lubaloy loads circa 1928/9. The 12-gauge 3-inch
Magnum load with 1 5/8 ounce of shot was introduced in 1935 along with the Winchester Model 12 Heavy Duck Gun. In the January 2, 1958, Western Cartridge Co. price list. the Olins introduced the 1 7/8 ounce 12-gauge 3-inch Magnum load.
January 2, 1958.jpg
A bit of real trivia -- From its introduction the Super-X 12-gauge 3-inch 1 3/8 ounce load (P37) was just that and the Super-X 12-gauge 3-inch 1 5/8 ounce load (P38) was the Magnum. February 19, 1952 --
February 19, 1952.jpg
By 1953, Western Cartridge Co. was changing their load symbols and along with that the 12-gauge 3-inch 1 3/8 ounce load gained the Magnum appellation. January 16, 1953 --
January 16, 1953.jpg