The introduction of the 3 1/2 inch Magnum-Ten was the first use of the term "Magnum" I find in North American shotgunnery. The 1 5/8 ounce 2 7/8 inch 10-gauge was called the Super-Ten. The next application of the term "Magnum" was in 1935, when Winchester introduced their Model 12 designed for the 3-inch 12-gauge shell and Winchester/Western upped the payload of some of their 12-gauge 3-inch Super-Speed and Super-X loads to 1 5/8 ounce. These 12-gauge 3-inch 1 5/8 ounce loads were called Magnums.
The 12-, 16- and 20-gauge "Short Magnums" were introduced in 1955.
The first commercial listing for a 3-inch Magnum 20-gauge was the Magnum Model 21 in the January 2, 1954 Winchester catalogue. However, there are no 3-inch 20-gauge shells listed in the January 2, 1954 Winchester Ammunition catalogue and price list!?! The Olin brothers and their buddies had been playing around with 3-inch 20-gauge Model 21s for some time before that. In the 1955 Winchester and Western ammunition catalogues the 20-gauge 3-inch Mag. is offered with 1 1/8 ounce of Lubaloy #6 or 1 3/16 ounce of Lubaloy #4. The 3-inch 20-gauge Mag. offerings from Winchester and Western remained the same through the 1959 price lists. For 1960 the 3-inch 20-gauge magnum with 1 1/4 ounce of chilled shot was added to the offerings, while the Lubaloy loads were still 1 1/8 and 1 3/16. The Winchester and Western offerings in 3-inch 20-gauge shells remained the same thru 1968 where my old catalogues play out.
In all of my ammo catalogues from 1900 to 1931, the only other length 10-gauge shells I find are 2 5/8 inch brass NPEs for hand loading. I don't find any loaded 10-gauge shells offered in anything but 2 7/8 inch cases.
