Hole in the front of the cylinder head.

"The one and only air injection port in air injection-equipped '72-up Slant-6 heads is at the rear, on the manifold mounting face, aft of the rearmost exhaust port."

yes, but how do you think the air injection hole at the rear face of the head connects to the exhaust ports?
the exhaust ports are connected to the air injection passage via a gun drilled passage that runs the length of the head. The cup plug that you identified closes the gun drilled passage and is the point at which the lengthwise drilling operation started.