Sideways "S" Hallmark in a diamond

by Ashley
(Bendigo, Vic, Australia)

A grade 18ct yellow gold 750, has a distinct horizontal S in a diamond. What does the S mean? There is also a pi marking in a circle with LG 75 WM (engraved on an intaglio crystal). What does this mean? If anyone knows where it's from or the maker and approximate age it would be of great help.