Hmmm I don't remember that hole. There could have been a locating dowel pin in there at some time. Or maybe someone tried to incorporate a vent (if both ball bearings have shields covering between the races on both sides, it should have had a vent). Unless someone else says otherwise here, plug it.
I dont have a T10 extension (tailhousing) handy to look at.
Anyone out there have a T10 tail they can eyeball for me?