I picked up a 1985 Cannondale, but can't figure out if it's a ST400 or ST500. Is there a way to determine model, there is no marking on bike except Cannondale on top tube and frame sizing on right rear seat stay. Serial number is 21093085177 on left chainstay. It has Shimano 600 components with half-step chainrings. Light blue in color. Will have pics up tomorrow morning.