Series IIa early cylinder head - port on rear?

Hello,

I'm slowly going through a '62 Series IIa that was originally a Central American vehicle. Having the cylinder head freshened and a while pressure testing, we found a leak in a large port, approx 1-5/16" in the rear of the head. There was an aluminum threaded plug that was pretty badly corroded and leaking. The threads are badly corroded as well.
Attachment 13737
(sorry, the photo keeps flipping upside down, I've tried uploading and flipping the image 180 deg and re-uploading)

What purpose does that plug serve? Asking in order to decide whether to chase the threads and find another plug. And I'm curious to know.

Thank you in advance,
Dan.

Good luck finding a tap that size. One possibility would be to have the hole machined out to the same size as a readily-available cup type freez
e plug

The parts book lists it as a "core" plug . The core is the sand pattern inserted into the mould to form the waterways in the head as the iron is poured in. Parts of the core have to project to the outside of the casting to support it in its correct position within the sand mould and also get the sand out once the iron has cooled. At the front there is a suitable hole , ie the outlet elbow hole, but the back there is nothing suitable. Then the core sand is knocked out of the water ways it leaves a hole in the casting which has to be plugged ( hence Core plug) . As suggested, if the thread is beyond repair , bore it out to accept a suitable cup plug