Thanks mate. You see that little metal plug in the middle of the head, right down near the gasket line? It's partly covered by No. 2 spark plug wire.
That's to plug the drilling that carries the oil from the block to the rockers & is unique to the A10 & 1200 engines as they simplified the oil passages from 1974 onward.

The double valve springs confirm it.
This is a 1200 GX cylinder head.

It's possible that somebody bought a whole 1200 GX engine & fitted it, or the top end of one & fitted it to a stock 1200 block.
If it's a whole GX engine, then the original GX fuel pump is gone [slightly higher fuel pressure] & the GX distributor has a different advance curve while the GX cam is a little more 'sporty'. Most GX models had flat top pistons, so pull a plug & with a small dimeter torch have a look at one piston when close to TDC. [probably best done at night or in a dark shed]
If it's a flat top then it's a whole GX engine, if it's dished, then it 'might' be a GX engine, then again, might not.

That should perk up some interest in your ute.

EDIT
Another clue. Have a look at the fitting on the water inlet pipe on the block. If it looks factory, [& I think it is] then it's most likely to be a whole 1200 GX engine as this was unique to the GX & is essential to allow proper manifold heating.