Huvius,
You may be correct on the sight attachment method and I will look and let you know.

I do not think the action was ever on a British or European gun because of the total lack of any marks or proof marks. If you read the whole thread you will see pics of another known G&H Farquharson which has the same engraving, almost identical. G&H serial numbered and marked their guns on the barrel, as you probably know.