Hole in the front of the cylinder head.
Does driving it open hurt anything then?
start the motor and see if you can feel exhaust pulses at the hole,
(if you have a mechanical fan blowing it will be hard to tell,,,but I would expect if it was leaking exhaust gas there would also be traces of carbon / soot around the hole) if you do not, either
1) the passages have filled with carbon over time and have plugged
and / or
2) perhaps the head was rebuilt at some point in time and the small passages in the exhaust port that lead to the lengthwise passage were threaded and had a small allen screw threaded in, that is a common practice.
if you do feel exhaust pulses, or see carbon, you have a carbon monoxide leak under the hood of the car, you would want to address that.
