U got one hell of a deal!
Not a franken bike. In 1990 the entry level touring bike was a ST400 and came with cantilevers. The first and only year as it was eliminated from the line up in 1991. Suntour 6 speed freewheel, 27" wheels, Suntour XCM drive train and came in Blue. The rest of the specs; sanaandterry.com/cannondale/year/1990/1990.pdf
Basically they used the same frame for all their touring models in 1990 and just changed the components for each model ST400-600-1000. I have a later year T-600 and it is the best touring frame and the only bike I own that is not steel.