Low car diet
Join Date: Aug 2007
Location: Corvallis, OR, USA
Bikes: 2006 Windsor Dover w/105, 2007 GT Avalanche w/XT, 1995 Trek 820 setup for touring, 201? Yeah single-speed folder, 199? Huffy tandem.
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This is my understanding.
Originally there was just 4500, which was marketed as double and triple compatible.
Then, 4501 (double only) and 4503 (same as 4500, but marketed as triple) replaced 4500. I believe this is because some people were breaking their shifter when they used the 4500 on a double setup when using the first two shift positions (inner and middle). Attempting to push the shifter into the outer position, which is locked out by the high limit screw, has the potential to stress and break the internal mechanism. Some people will set up the shifter so that the middle and outer positions are used, but this might mess up trimming.
In short, the 4501 can only be used for double. 4500 and 4503 are effectively the same and can be used for double or triple, but be careful if using a double.