shift and kickdown levers for A904

I don't know how many there are, but there are several different ones. They varied by application. Just for an example, if you convert a slant six with a 1 bbl carb to a super six 2bbl, the kickdown lever is different.
Those are not part numbers. They are a reference number. For example in the 1972 parts manual there are 7 different part numbers for 21-142-12 depending on vehicle, engine and trans. For 21-142-18 there are 2 different part numbers depending on whether 904 or 727 trans.