[Researcher]

You have to go back quite a ways to see the A and B shells. This from the 1880 Union Metallic Cartridge Co. catalog --

1880 UMC Catalogue.png

and 1885-86 --

1885-86 UMC catalog pages 12 & 13.jpg

mention A and B sizes.

By the 1887 UMC catalog there is no mention of A and B sizes --

4-gauge 1887 brass.png