Welcome to the Forum.
If it is an English item, then there are quite a few possibilities. ''S & S" would be quite a common combination because it would include every "S" firm with the " & Son/s" in their title. However, the abbreviation "Stg" for "Sterling", is not that commonly used in England, but is indeed very popular in Australia. Stokes & Son of Melbourne is the stand out name. I'm not saying this is by them, or even that it is Australian, but offer it only as a route for further research.
Hopefully others may have their opinions.