Tom,
Check out the SORI Thread on the SSS home page for starters. The The Roy M. Marcot book, "Spencer Repeating Firearms", devotes Chapter 15 to the Spencer Shotgun. Chapter 16 has info on the Bannerman Spencer Shotgun.
Book is very pricey, now.
Email me at fsgrand2@fairpoint.net and I will add your alias to the membership list.
Two Flints