396 Head Question

I had a set of 396 heads getting new valves and porting. Due to problems, we used another pair of heads. These other heads have a outlet not on my original pair. The threaded holes are between the exhaust outlets, and just below the valve covers. I will plug them, as they enter into the water jacket area. I just wondered what their purpose is.

Water temp sender, one on each head.

Makes sense. I have my water temp sender mounted on the intake at the outlet port. It would be interesting to see if there was any difference between head temp and temp where the coolant enters the radiator.