I know, "bolt on " is a pretty loose term. Where does it start and where does it stop. If there's a competition going on, the limits need to be mentioned.
i think a fairly reasonable interpretation of "bolt on" is: installed with basic hand tools and no machine work.
if some pendantist wants to argue that: well technically a crank is a bolt on, they're more than welcome to do so while i shove a oven mitt down their throat.