Hi,
Neal and Back's "Great British Gunmakers" series includes 3 books. "GBG 1740-1790: the History of John Twigg and the Packington Guns", "GBG 1740-1790: Messrs Griffin and Tow, and W. Bailes", and "GBG 1540-1740". The lock on your gun, with its roller frizzen, is from the 1770-1790 era. The first book I mentioned is probably the best resource for you. I tend to focus on British guns from the 1740-1770 period so the second book by Neal and Back I listed is my best resource, although the Twigg book is also helpful. As I move back in time to the era of Dolep, Reynold, Truelock, and Barnes, the last book I listed is my best resource. Neal and Back also wrote 2 books on the Manton family of gunsmiths, another on Forsyth and the percussion revolution, and finally a book on British gun cases and trade labels. All together, those books comprise perhaps the most comprehensive survey of British gun making during 1540-1840. If you add John Burgoyne's book on Queen Anne pistols, Norman Dixon's on Georgian pistols, and Atkinson's on British dueling pistols, you will have a pretty comprehensive library on British firearms. The emphasis is on sporting guns and books by other authors like Bailey, Blackmore, and Goldstein cover British military firearms.
dave