Still niggling away at the things I don't know about the Cutler...
I'm getting the impression that the maker was a "follower" (for which read "poor imitator") of Manton's style, but suppose that once Manton had set the standard, that description might be applied to most English gunmakers of the later 18th and early 19th centuries.
For example, here's a picture of the trigger guard of a genuine Manton:
And here's a picture of the same area of Cutler's gun:
I'm less interested here by Cutler's emulation of Manton's pineapple finial: though it's a pretty good copy, it's too widely imitated a motif to put much weight on. What interests me more is Manton's depiction, on his trigger guard, of a pointer flushing a bird from a clump of vegetation; and how this is emulated (very badly) by Cutler: indeed, Cutler's engraver, perhaps doubting his ability to depict the pointer's legs, portrayed it lying down:
Ahead of it is a bird rising from a clump of vegetation:
A similar thing occurs on the tang of the butt-plate. Where Manton (on another gun) has a nicely-realised trophy of arms:
Cutler gives his customer this approximation:
Anyway, as much fun as I'm having looking for stylistic similarities, none of this resolves the "Stubs twist" and "London" questions, or explains the lack of proof marks, or helps assign the gun to either Cutler's Lench St. (1807-1811), or Weaman Street (1822-1840) periods, or whatever he was doing between 1812-1821... although, I suppose absent proof marks may be more likely in 1807-811 than in the 1820s and after.