Jules, that is a very handsome flintlock (the photograph of it do not display on my screen this evening - might want to repost). I do not believe that it was mandatory to proof barrels in the UK until the Firearms Proof Act of 1813. You mentioned this earlier. Here it is and it created the Birmingham proof house at the same time:
https://vlex.co.uk/vid/firearms-act-1813-808157429

If so then surely your Cutler is pre 1813 or if it dodged the proof laws a fortune in fines awaited the maker. There are examples of gun makers who were so sanctioned. And thanks for your highly interesting postings on the early use of Damascus in Liège and its gradual infiltration into UK barrel making. Fullerd was the last London barrel maker and he closed shop in 1843.

I may be wrong but if so Diggory Hadoke is as well for his widely used proof chart begins in 1813.
[Linked Image from i.imgur.com]