I did a scan of the available Union Metallic Cartridge Co. catalogs/price sheets/broadsides. The 1873 price list offers brass shells from 8 to 14 gauge. The 1877 E. Remington & Sons catalog offers both A & B 10- and 12-gauge brass shells and 16B brass shells. I only found No. 11 brass shells offered in the 1880 and 1882 (No. 11B) UMC catalogs.
1880 empty brass shotshells.png
Everything 1884 and later no No. 11 shells. Nothing I've found offers a No. 11 paper shell.