Just to add some info:

1st generation: The original 600 group (6100 series) was introduced in 1976. It was significant as it was the first complete group set for mid level bikes. I think it was also Shimano's first group for Campagnolo dropouts (bye bye Simplex). This was the first group set that Shimano began selling in volume around the world.

2nd generation: The Shimano arabesque group, real name 600 EX or (6200 series). Every time I hear "is 600 EX better than Arabesque?" it bugs me a little because the Arabesque is indeed an EX group, just older.

3rd Generation, Shimano 600 AX. Yes, I know I said it was produced concurrent with the 2nd gen EX but this one was 1981~1984. This is an interesting group; think franken delta brakes and a cable hanger on the derailleur that makes it looks like an alien earpiece.

And finally, 4th generation 600 EX, 1985. The hot second Suntour's slanted parallelogram patent expired everybody redesigned their stuff. The 4th generation finally implemented said slanted parallelogram.