What 1990's bike have I bought?
That doesn't seem to match any Cannondale serial number formats.

https://vintagecannondale.com/info/serial_numbers/ says that there was a non-drive side placement for serial numbers, but they don't start with S.

That said, it seems to be a decent-enough looking road bike.  Simply add yellow paint to cover up the scratches, or perhaps contrast-paint over the damage with a different colour like deep black, or red, or ride it as-is.

The small support for the gear cable, lower on the head tube, is somewhat distinctive, and someone might recognise that.  Its not a cable-stop, it is just holding the cable outer.