Several years ago I tried to collect info about the X & O stampings on the '63/'64 9770716 Head castings.
Never was able to collect enough data to come to a conclusion.
I did speculate based on what I did find and have mostly forgotten if any data contradicted what I came to believe.
It starts by the knowledge that PMD used these Heads for the '64 421HO and also for various '64 389 applications, including the '64 GTO.
All had an advertised 10.75 cr. The advertised cr did not especially match reality, but it made no sense to me that the 421HO Heads could be used directly on a 389 and yield a higher cr than the 10.5 cr 9773345 "regular" 389 Heads without some milling.
b-man, you may be right about the valve relief accounting for the chamber volume difference.
But I was never very "satisfied" by the various explanations that suggested larger vol reliefs used for the 421 pistons and/or head gasket thickness differences.
Especially since the Heads exhibited the stampings.
Then there was the fact that the early '64 GTO Heads made use of the lighter duty Valve Springs, switching to the 421HO Valve Springs around Nov. '63.
I have a pre-Nov Head example which I believe was original to my own Nov. built GTO which was stamped X X and a later '64 Head that was stamped X O (not original to my GTO).
My speculation is that the early X X Head indicated a Head milled for 389 use with lighter Valve Springs.
And the later X O Head indicated a Head milled for 389 use with heavy duty (421HO) Valve Springs.
What I was never able to find were examples of 9770716 Heads known to be original to a '64 421HO to see how they may have been stamped.
Logic suggested that they wouldn't be stamped at all, only those reserved for 389 use would have needed to be specially milled and given secondary stamps to identify them.
Also, IIRC, the 9770716 Head was only used for the 421HO in '63, so no reason for any stamp codes on them (if they only came in one flavor), but finding examples from '63 proved impossible for me.
Just never could prove whether my speculation was valid.
The '65 77 and '66 093 Heads also had shared 389/421 usage too but I never heard of anybody claiming to have seen secondary stamped codes on any of them.
The '64 X and O stampings can be found on the dime sized bosses on the center exhaust ports.
The biggest problem is corrosion of the casting which makes detection of secondary stamp codes difficult.
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