My book on .410-bore shotshells say Remington Arms - Union Metallic Cartridge Co. introduced them in 1914. No .410-bore shells were shown in the 1913-14 Remington Arms - Union Metallic Cartridge Co. catalog, they first appear in the 1915-16 Remington Arms - Union Metallic Cartridge Co. catalog --
1915-16 Catalog Small Gauge Loaded Shells.jpeg
2-inch .410-bore Remington Arms - Union Metallic Cartridge Co..jpg
Those remained the offerings through the 1923 Remington Arms Co. Inc. catalog --
1923 .410-bore loads.jpg
2-inch .410-bore Remington Arms Co., Inc..jpg
These loads were 3/10 ounce in the 2-inch shell and 3/8 ounce in the 2 1/2 inch shell.
The .410-bore shell first appears in the 1916 Winchester catalog offered only in 2-inch.
By the mid-1920s progressive burning smokeless powder was applied to the 2 1/2 inch .410-bore shell --
Super-X .410 3-8 oz. 7 1-2 Chilled Shot.jpg
Shortly after the 3-inch .410-bore shell was introduced in 1933, our manufacturers upped the payload in the 2 1/2 inch shell to the 1/2 ounce we know today.
NITRO EXPRESS .410 half ounce load.jpg