Antique Moonstone Cufflinks with British Hallmarks

This pair of cufflinks have round cabochon Moonstones in a silver setting (indicated by two incised hallmarks). They have a swivel toggle the lays flat, usually called a Whaleback. No makers name, other than the hallmarks which indicate UK, Circa early-mid 19th century. Though very tiny, the "duty" mark appears to be the head of King George III ( there is a "K" on the left and a "3" on the right of the image). The other hallmark is that of origin but is very difficult to read : I believe it is probably 3 Castle towers (Edinburgh ?). The moonstones are set in a bezel with a decorated collar with original patina. Very interesting appearance with no chips or cracks.