I just had a look at my original 73 318 intake manifold.(cast in 72) It has that extra hole in it as well. I'm not sure what it's there for. Maybe they needed it there for some machining process at the plant? Nothing goes there. There is nothing on the cylinder head in that location.
I also have an 80's thermoquad intake with the smaller style thermostat housing. That intake does NOT have that extra hole in that spot.