Hello SSS,
Thought you'd be interested in seeing a Billings-Spencer, as shown in the Marcot book, of a Single shot 12 GA shotgun. It is currently on sale at GunsAmerica with this seller description:
This is a rare Billings-Spencer single shot 12ga shotgun. After the civil war Specner became Billings-Spencer, then just Billings( tool makers) This 1867 to 1870's shotgun has no markings, or numbers. This is the second one I've had in thirty years. Very few were made. Has some flaking on the nickled reciever, barrel is full blue with surface corrosion from storage. Bore will clean to good to very good. Wood very solid black walnut , no breaks. splits, or repairs. Action works like the trap door rifle, by pulling back the knob and lifting the breech! Two Flints