Moon and Star Pin | What does SL18 Signify?

by sue levy
(leigh, Essex UK)

antique Cravat pin, circa 1880

It is a cravat pin of a moon and star with pearls and diamons,the weight is right for gold and when scratched there is no discolouration, it is gold beneath the scratch.
the markings are a raised square with the letters and numbers of SL18 in the square, all in relief.








Where are the markings?