From SASS Shooters Handbook, TENTH EDITION (VERS 10.4), April 2004: "Any side by side or single shot shotgun typical of the period from approximately 1860 until 1899 without automatic ejectors, is allowed."
From NCOWS By Laws: "Various hammerless side-by-side double-barreled or appropriate hammerless or exposed hammer, top-lever break single shot shotguns that retain the characteristics of pre-1899 originals, e.g.: without ventilated ribs, modern recoil pads, white-line spacers, etc."
For SASS the auto-ejectors have to be disabled. I am not sure for NCOWS, though.
Slim