Just guessing here but I think that when the head was reworked for the replacement seats it got hot tanked and there was no metal left in the bottom of that hole. I would just tap it and put a set screw in there with a sealer and make it flush or just a bit below and forget it. There are water cavities in there for cooling. So just close it up and you should be just fine. You won'y hurt any thing by closing it off. As for why there is a hole? Whitedog has as good a guess as I could think of. Might have used a pin to keep it all aligned. There would have been a corresponding hole in the manifold but who knows when or what manifold was on there. Like I said there would be no harm in closing that hole up. Cost? about $1 and your time. Gotta buy the set screw.:)
