Bill, it is hard to be sure without seeing a photo but it sounds as if the Lancaster may be an early one based on Needham's 1874 patent. This is believed to be the first Brtitish patent for a properly working ejector system. The mechanism for this ejector sytem was actually in the rear barrel lump which was much larger than usual and projected beyond the floor of the action.
There is a photo of a Lancaster like this in Nigel Brown's "London Gunmakers". The gun is serial number 5331 made in 1883. It also shows a very elongated lock plate, just like a back action hammer gun.
If you can get a photo posted it would clarify if this is correct or not.
