What 1990's bike have I bought? I've not had a road bike since I was about 16 which is too many years ago to want to remember so forgive my ignorance.   I can remember that Cannondale were said to be good back in the day so I've bought one locally.  It was a fixer-upper so reasonably cheap.

I noticed having got it home that where the paint had chipped the colour underneath was a different colour. What's more it looks like a bit of surface rust.

Alarm bells are ringing and I'm wondering what I've bought.  I've spent hours trawling the web but failed.

This bike is said to be at least 20 years old. The bottom tube is tear-shaped and is slightly larger than the other tubes.

The other thing that makes this stand out for me is that the top tube has rolled at both ends slightly but 90 degrees from the other end. This means when you look at the bike from the side the top bar looks thinner, has a lower profile where it attaches to the head tube, whereas the opposite where it joins the seat post tube so from side on it looks wider, if that makes sense. 

The serial number is S followed by about 7 or 8 digits and runs from back to front on the crank drive side. The forks are unusually straight too.

Can anybody put me out of my misery please?
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.