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600
Shimano's second-from the top "road" series used to be called "600", then it became "600 Ultegra" and eventually the marketeers dropped the "600" designation.
The 1986 600EX group was the second to receive S.I.S. indexed shifting shifting, after original Dura-Ace. Shimano "trickled down" S.I.S. to the 600EX group with a longer cable travel, which became the standard for the rest of the Shimano line. This led to a decade of incompatibility between Dura-Ace and the rest of the Shimano line, only resolved in 1997 when Dura-Ace went to 9-speed and lengthened the cable travel to match the rest of the line.
The 6-speed 600EX shifters had an unfortunate design flaw: The indexing unit had 5 tiny tabs that engaged notches in the bushing that fits against the down tube. These little tabs are fragile and by now most of them have broken. They don't generally break all at once, but as they start to go the shifting gets more and more erratic. There is no fix for this except to replace the shift lever. This is a good time to upgrade to 7-speeds or more.
See also Shimano Models
