The way I understand it was they first released the original slow down kit for 21s and early 2388s, which consisted of a larger pulley behind the top variable fan drive pulley, longer fan jackshaft belt, and shorter variable fan belt.
Then, at some point in time, some engineer decided that it was TOO slow for certain conditions, so then they released a slightly smaller shaker pulley (the big one behind the elevator) to speed it back up just a tad. So in reality, there are at least three different speeds of shaker systems roaming the countryside.
What I don't know is serial # breaks, as to which machine came from the factory with what kits & pulleys & updates. As with most axial flows, serial # doesn't mean much when you order parts, there's been so many different updates and changes through the years, some machines got them some didn't, so you just have to play dectective and figure out what you have and compare it to what you need, and sometimes the corresponding parts to go along with it.
Clear as mud?