Back in the 1880s when 42873 was made, the big companies like Union Metallic Cartridge Co., Winchester, etc. offered empty brass and paper cases
1885-86 UMC catalog pages 12 & 13.jpg
and a lot of shooters were stuffing all kinds of loads in brass shells for themselves.
Loaded ammunition came from independent loading companies like Chamberlin
Cartridge Co.
1886-7 pg 5.jpeg
1886-7 pg 6.jpeg
1886-7 pg 7.jpg
Note the "special" loads specifically for Parker Bros. guns of that era.
Most metropolitan areas had companies or individuals that provided loaded shotgun shells. Here in Spokane we had Ware Bros.
1901 No. 10 Ware Bros.jpg
Over in Portland, Oregon it was Honeyman Hardware, while in Seattle it was Piper & Taft and Hardy Hall. The big loaders in California were Selby and California Powder Works.