Raimey,
I have several sources, but the best single source is an article for an Arms & Armor Society written in the 1950's. If my brain was working right now, I could quote chapter and verse, but I just can't get it out of the cob webs. One book (awesome book) I can pull off the top of my head is Early Percussion Firearms, by Lewis Winnant. There are also a couple of books by Merrill Lindsay with some good information. In his book 100 great guns (IIRC), he has a photo of a Robert made Pauly in 20 bore.
George Hoyem (whom I've had some conversations with) has covered the Pauly cartridges in one of his books.
I have a friend who has not only seen, but handled the two guns in the Royal collection, one of which was made by Durs Egg.
