[Researcher]

Other than the one Parker Bros. 24-gauge made for United States Cartridge Co., the bulk of the 24-gauge guns made in the U.S. were single barrels.

The Remington No. 3 --

Model 1893, No. 3 1899 Catalogue.jpg

Harrington & Richardson Model 1905 --

Model 1905 from Catalog #10, 1908, page 27.jpg

and later Model 1915 --

1920 catalog Model 1915.jpeg

One hundred plus years ago the 24-gauge was loaded with 3/4 ounce of shot --
NITRO CLUB 24-Gauge Infallible No. 8.jpg

REPEATER 24-gauge #6 02.png

While the 28-gauge came with 5/8 ounce --

NITRO CLUB 28-gauge, X25D7C.jpg