I really bought my gun for its 32-inch 20-gauge Vulcan Steel barrels that were fitted in the late teens or first half of the 1920s, after the rib legend changed the CT, U.S.A. but before the Parker Bros. Overload Proved stamp. The original 12-gauge 32-inch barrels were whacked two inches and are straight cylinder. As was Parker Bros. practice in the 1880s and into the early 1890s they are very overbored .757". I have used them shooting Skeet with my Remington STS 7/8 ounce reloads with good results.
In the 1870s and 1880s Union Metallic Cartridge Co. and Winchester didn't offer loaded shotgun shells. They provided brass and paper cases and shooters loaded their own or local smaller operations loaded them. Note these offerings from Chamberlin Cartridge Co. that Load No. 56 and 57 are specifically for Parker Bros. guns --
1886-7 page 6.jpg
1886-7 page 7.jpg
Also in the 1870s up to the middle 1880s brass cases were offered in two types -- A for use in guns chambered for paper shells and B for use in guns chambered to use brass shells only. By the 1887 UMC catalog they no longer mention A or B brass cases.
About 1890/1 both UMC and Winchester began offering factory loaded paper shotgun shells. UMC offered the option of smokeless powder from the beginning, while Winchester provided smokeless powder shells to selected shooters in 1893 and to the general public in 1894.