The difference is because your 56 is what is called a loadamatic - which means "crap"amatic... just kidding, but it is a lousy distributor. I has only vacuum advance and was built to match ONLY carbs that will send it the right signal. Any carb from 1957 and on will NOT send the right signal to the 56 and earlier distributors. If you are switching distributors, modify your carb to give the right signal or replace your carb with a 57 and/or newer model.
The difference is not a problem... it will slide down on top of your hexagonal shaft with no issues.