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I would throw Belgium or France in as possible countries of origin. The gun may have been made on the Continent and then brought into the UK for some finishing and proofing. Before the 20th century, I don't think the guns had to be proofed in their country of origin or marked as imports.
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Belgian makers produced a lot of parts for export. Enough of this trade went to England, that the English Consulate began producing estimates for their home office. However, even if the barrels were produced in Belgian and exported before they were joined, I believe they still had to be proofed. Joined barrels would have had the weight stamped on them in addition to the proof.
There may have been a way around this, but I am have seen evidence of it.
Pete