Originally Posted by
HillRider
I looked at the Shimano Tech Documents for both the ST-5600 and ST-5603 and see no difference. The ST-5600 is double/triple chainring compatible already so I have no idea why Shimano lists the ST-5603 as a separate component.
I thought it odd too. My best guess is that the designator regarding shifter is only relevant when they are sold as part of a larger gruppo, with 5603 reflecting the craskset, triple FD and the long cage RD that would come with a triple gruppo. When the shifters are sold alone, I don't know that there is any difference.