stg trident hallmark on antique earrings

[Nrgchik16]

Hello all. I've been trying to figure out the manufacture and approximate date of these little beauties. They are smokey topaz in a huge cradle. The screwback is marked STG which I know might mean it's Australian, bug I can't find any comparable images of the hallmark next to the letters. In my opinion, it looks like a trident or a U and a W on top of it. It's small and I'm hoping my images clearly show it. If you have any ideas, please let me know. Thanks!