Ddgonzal
I think i might have it backwards. I know that the A14/15 heads have three of these coolant holes, but i thought the single one was at the rear. It looks like i got it arse about.

That center hole is normally covered by the manifold & the gasket.

The GX has four of these holes. Is that the full drilled size of that coolant hole, or is it partly blocked?

Whats the story with the rest of that A12a engine. Is it a junker, or will it be rebuilt? I think you know what i need from it.

The factory race option inlet manifold mounted two solex carbs, much like Webbers, & obviously the manifold had the oval ports, but was drilled with the earlier, center stud, manifold stud pattern.
I do not know if any other manifold was ever offered with this stud pattern & port configuration.
GX heads came with the extra holes, just in case one wished to fit this manifold.

Do the A14/15 heads have this extra hole?
The GX also had an extra water outlet hole under the rear pair of inlet ports. A14/15's seem to have only 1 at this location.