John, see pg. 2, EUN 190810 was assembled 12-19-63.
That break point date is consistent with my '64 engine log;
EUN 163xxx has a block cast date of K283
EUN 203xxx has a block cast date of L233
EUN 225xxx has a block cast date of A64
Cast dates can vary but generally, I would say that 9770716 Heads cast after about L153 should generally all have gotten the HD valve springs at Head assembly assuming they stopped using the standard valve springs on or about 12-19-63 and on that date most of the Head castings would have been cast L153 or later.
I imagine some earlier castings would have been assembled after the break point, but not a lot. Every now and then, I found a block cast much, much earlier than the engine assembly date (established by the EUN). So it is possible that a rare Sept, Oct, or Nov Head casting wasn't assembled until after the change was made. But for sure, you should never find a Head cast after 12-19-63 assembled with the standard springs.
If the stamped codes indicate the springs used for the Head assembly, you would not find the standard spring stamp code on a later dated Head casting.
My assumption is that the XX would indicate the standard springs. So I would not expect XX stamped on later dated Head castings. If Bill's April Heads are stamped XX, that would blow that assumption up.
I never recorded Head cast dates for my log (since it is under the valve cover), just the block cast date. But you can see that the block cast dates were usually within a few days or less of the date the engine was assembled. From what I understand, the Heads were typically cast very close to the date of the block in many/most cases. So that is why I suggest L153 would be a pretty good guess of the break point.
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