Very nice piece, Auldjin.
Likely William Grice, Birmingham, 1766 -90. (Marked his guns London.)
The proofs appear private.
Going by the lock and trigger-guard, I'd say towards the beginning of his career, as the lock has the long scear spring, and the trigger-guard has no fillet to reinforce it where the bow joins the rear extension.
Both the short scear spring and the fillet were becoming the fashion by the mid 1760's.
Again, a Very nice and well preserverd piece! And yes, don't dig out the "Brasso"!
(I sometimes rub the brass with my hand, just to highlight it a bit, but no polish.)
Congrats!!!