28 Unknown Hole

On the 'F' head super six engine, on the horn side (opposite the water pump), there are three freeze plugs. Often the third, closest to the fire wall, has a fitting for the temperature gauge.

Directly under the first and third plugs, at right angles (meaning vertically), under the casting edge are vertically tapped holes. Currently, they are open in my car, and so I need to plug them to check for catastrophic cooling leaks, hoping that there are none.

My question is simply, "Why are they there, and what is supposed to go in them?"

Please let me know.....