Those are Ordnance private proofs. They were done on Tower wharf, for the trade, in the Ordnance proof house. This is one of the questions I'm working on with DeWitt Bailey. His research suggests that the Ordnance ceased doing them very early in the 19th century, probably before 1805. With the Napoleonic wars raging, they had little interest in encouraging private trade. In fact, they were doing everything to discourage it so that the gunmakers would produce government arms. A date of 1790-91 is completely consistent with those marks but a date of 1816 is not. After the establishment of the B'ham Proof House in 1813, private proof also disappeared and everything that was proofed was done either in London or B'ham.