Well there used to be 4 vesions, from what I can remember the differences were.
Sterling Bronze was the site gun, fixed barrel and basic parts.
Sterling Silver had a removable barrel.
Sterling STP had all the parts polished and matched, better barrel, bottomline and hose.
Sterling Stocker was a Silver with the horizontal feed.
If I remember right, in the old days the STP was the only one that came in a choice of colours.