Bill, thanks for clarifying your stamp marks.
The fly in the ointment would appear to be the early XX stamped Heads.
If all late 9770716 Heads were the same and if at least some of them were stamped XO (notwithstanding Bill's example lacking any stamped marks and his example that was stamped off center so that the marks aren't clearly interpreted), then logically the early heads stamped XX must have been intended for the 389 GTO (the '64 10.75 cr 389 big car tri-power engines are way too rare to know whether they shared the GTO Heads or the 421HO Heads early year but since there is no mention of early & late valve spring choices for this big car engine, I suspect it should have gotten the 421HO Heads and not the GTO Heads for the entire year).
But the only known 421HO example is b-man's and it is stamped XX. Either my interpretation is wrong or else that early '64 421HO engine was incorrectly assembled with the GTO Heads having standard valve springs.
b-man, if you have the tools to check it and if the valve springs on your 421HO Heads are still the originals, you could measure the wire dia. for the inner and outer springs to see if they match the standard or HD spring dimensions.
That might help point to a conclusion.
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