Cannondale introduced their first bicycle in 1983 and it was known simply as the Cannondale aluminum bicycle. In 1984 it became the Cannondale Sport/Touring bicycle when the Cannondale Mountain bicycle was added to the line.

The model numbers came in 1985, as described by Walter, with the addition of SM prefixes for the mountain bike models. My literature shows an ST400, ST500, SR300, SR500, SR900, SM500 and SM600 for that year. After that, things really started to proliferate.

As Walter states, the frames within any prefix were essentially identical and the differentiation came via the components. Perhaps if you could remember some of the component models, then we could identify the bicycle model.