Another thought - BCS in Bend last year had a newer-than-my Beaver in the back lot which was going to be parted out. I believe it had a fridge fire and the coach was totaled. They could have one in the Parts Dept and/or have one from the parted out coach.
That part, in fact the whole wiper system, has to be a common part found on many other vehicles - probably trucks. If you could get the bad stud out so you could see it, then you might be able to match it to a common wiper system - similar to many other parts and systems found on Beavers that come from common systems found on other vehicles and trucks. Also, if you get in there and see a manufacturer name on the motor and/or actuation arm between the motor and stud, then perhaps you could match it up.