I mean the factory 2 speed bells that have the gears already on them.
Here is a list of the bells T15-18 G84307-04, T14-17 G84308-04, T14-18 G84310-04.
I only have one 15-18 factory CEN bell and it is steel and it apears to be unthreaded but I don't want to mess the gears up to see if it is. I read somewhere that only the 11-14 bell is threaded and uses aluminum pinions and all of the others are one piece steel units with the gears permanently attached and all of the CEN bells that I have seen, this has been true so far.