b-man, that is excellent and I think is pretty compelling "proof" of the actual difference between the XO and XX heads.
Since the early GTO Heads are the only version identified in the published PMD documents that were supposed to have used the standard springs, my guess is that your 421HO was built with the XX Heads by mistake. No indication that the 421HO Heads were ever supposed to use the standard springs and given that they were a carryover from '63, very doubtful the standard springs were intended for the 421HO Heads.
It even occurs to me that the existence of 2 flavors of these Heads made such an error probable and may have been the reason to change the GTO Heads (rather than any failure problem that may have materialized in the first few months of production).
Wonder how many other early '64 421HOs got the XX Heads (and for that matter, I wonder if any early '64 GTOs got the XO Heads).
Had given up trying to discover what those stamped codes meant. Neat to finally get some compelling evidence.
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