Originally Posted by Chuckk

Check out the rear dropout. No fender/rack mount and the shifter cable runs on the top of the wheel stay. I don't know if it's a big frame feature, but the seat stay arm of the dropout is unusually long.

And here's a story to go along with it. A buddy of mine bought an old Campy black Cannondale many years ago, and I'd go compare it, but it's now under loads of stuff in his shed.
While he was still riding, a tech came past to do cable or something at his house and saw his Cannondale. He claimed to have worked for Cannondale racing in the early years, and he kept telling him, "That's a special bike, it's special." They looked all over for serial numbers, and there were none.
No relation to anything, don't know much, but the bike may just BE a little special.