Guessing we are talking about Dolmars here?
From what I can gather (not that I know much about Dolmars) not much more than a decal sticker.
They both seem pretty identical bore/stroke, specifications in general.
Both seem to make the same rated power, both seem to run the same carb (though not sure if the 133 was governed?).
Maybe the cylinder porting differs on the S model? Maybe chain brake was standard on the S- sometimes it can be that simple.