I don't think it is an overflow hole bolt.
I believe the bolt tightens down on a slotted area on the top of the spindle inside and thus is supposed to hold it in place.
I seem to remember this from my 75, but I don't know if prior years had this "feature"?
With patience, I have been able to bet that top bolt off, sometimes by removing the rear fender to get to it.
I used to put the oil in there, but nowadays I take off the right side end cap with the bike on the sidestand so it is tilted to the left,
then shoot a load of oil in there with a syringe and put the end cap on fast.
Aren't the bushings supposed to be impregnated with oil anyway, I suppose the oil is to slowly seep over and around the spindle to help lubricate
it against the bushings.