I run my mechanical temperature gauge sender there, or the other side. They usually had a thermal switch that opened vacuum to something once the engine warmed, or close it off after a certain temp was reached. On a 68/69 it might have been the heat riser door in the air filter housing, those flaps in the snorkel would open and close to allow heated air from the exhaust to warm the intake to help with cold weather drivability. Later engines used those for part of the EGR system. I have a couple of those plugs in the shop and a bare intake I just had cooked, if I remember I will measure the hole so you can plug it with a pipe plug if you want.