It is a later model of the same car. It is an Alfa Romeo Spider. The earlier version that you saw in "The Graduate" had the rounded off tail, and the later models were more "cut off". In fact, later models of this car included "The Graduate" edition, with a rear spoiler. Essentially the same basic model.
As long as Fiat was mentioned, the Fiat Sport Spider 850 had a similar FRONT end, but was a much smaller car, and was rear engined.
After seeing the movie multiple times I start noticing the set decor and other props in more detail, afterall, it's the whole thing that comes together to make a great movie, right?
I felt it just HAD to be the Spider since they used the "Mrs. Robinson" song, but couldn't find any confirmation of that.