I think that only old, old gold coins and gold items claimed by the English or Spanish government, etc. had any sort of "marks" on them prior to say the 1800's..
So, one travelling the world could find all sorts of "unmarked" gold items if they got lucky. Especially "blackmarket" antiquity gold. Don't forget, the Spanish melted down tons of Aztec gold for shipment back to Spain and they didn't get it all.
It seems like every week now that somewhere around the world someone "digs up" a cache or stash of gold items.