Good catch. You are correct on the rear derailleur. Only the cranks, calipers and shifters are first generation. 600EX was introduced in 1978, but Shimano continued to manufacture the 1st generation as well. I'm not sure how long they co-existed, but it was for at least three years. Given the bicycle has a 1976 serial number, it would appear that at least the rear derailleur is aftermarket.

Historically, the introduction of the 1st generation 600 was very signifigant. Prior to this, the the only complete groups were Campagnolo Nuovo Record and Shimano Dura Ace, which meant an expensive bicycle. 600 brought the group concept out of the high end and started a trend that would ultimately lead to the demise of many European component manufacturers who tended to specialize in a single component and survived on low and mid-range bicycles.