It's South African and dates to the 1950's-1960's era.
The name of the maker is unknown to me, but his mark represents a lad by the name of Dick King who rode from Durban to Grahamstown to carry the message that Durban was being attacked by the Boers, he's a South African legend. The 'STG' indicates sterling and this is confirmed by the national mark of the springbok head. I'm not aware of a date letter system being used in SA, but perhaps that 'Z' mark can cast new light on this.
Now do me a favour, it would be great if you could post a clear sharp image of the mark for our records.
Hi guys - here's another example of the same silversmith's marks - this time with a date letter G or possibly a C- also on a set of three napkin rings. And like a lot of modern South African silver, what they lack in style or design, they make up for in weight.
regards Frank