Transmission stud
I have to do some looking around to see. If you are talking about the two holes at the front of the tailshaft where the transmission mount goes, I believe that they are supposed to be 7/16 coarse thread BOLTS.
Someone could have put studs and nuts in there for some reason though.
I may not be able to verify until tomorrow as I would have to dig around in my wood room to check out some transmissions.