3/8-12 shafts fit Bludworth
I have a 3/8-12 tap
Even though Blud is 3/8-11.5.
For instance a 3/8-11 shaft won't fit a Bludworth however I was able to thread a Bludworth shaft onto a 3/8-11 butt.
It seems as though the butts can handle a shaft that is a half step larger