Hi,
I am sure it was a Ferguson. There were quite a few civilian versions made including for the East India and Hudson's Bay companies. Eight shots is a stretch but Patrick Ferguson demonstrated 6-7 aimed shots before King George in 1776. The Ferguson has a fatal flaw in that the stock was very weak at the breech where so much wood was removed for the screw breech and lock. Quite a few surviving examples are broken at the breech. It was no "wonder weapon" . I built and shoot one quite often.
There were other breech loading mechanisms tried at the time including "tip up" styles so it could be another mechanism but I suspect they were describing a Ferguson breech loader.
dave