In the early years of the 20th century, many of our manufacturers offered single barrels and low priced doubles (usually cheap Belgian imports) in .44 Caliber. This is my Harrington & Richardson Arms Co. Model 1905 --
These guns were intended for the .44 WCF shot cartridge and the .44XL shot cartridge. See the bottom of this page from the 1906 Wm Read & Sons catalogue --
These guns were actually .44 caliber. The .410-bore spade for my bore mic rattles around in this H & R.
This from the 1912 SD&G catalogue --
The European 12 mm shot shells began being manufactured in the U.S., about 1915, as the .410-bore/12 mm, and J. Stevens Arms & Tool Co. claimed to be the first U.S. Company chambering for that shell. This from the 1918-19 Rem-UMC catalogue --
These early 2-inch .410-bore shells carried a load of 3/10 ounce of shot. When the 2 1/2 inch .410-bore shell was introduced it carried a load of 3/8 ounce of shot.
Threaded Mode