I believe at one time all shimano shifters were intended to use a round base but as the square base base, as the square one became more standard, if you will, shimano devised the little adaptor plates that are commonly found with index shifters.
do the screws with the 600 shifters fit the bosses? alot of the......
ah wait a second... what 600 shifters? are they indexing?
you need those little backing plates like on this photo below the shifters. if your bike is steel you can use either the curved or flat backed although the curved look better. for alloy you need the flat ones. check with the mechanics at your LBS who have been around the longest, you know the guy who never tosses anything out.
