[Barry_R]

X means nothing - it's a foundry identification scratch for the valley core
The 66-427 is similar and is found on 390s too sometimes
There is no such thing as "high nickel"

The 12 DIF means Dearborn Iron Foundry - essentially pre-'74ish
If the core plugs are thread in brass it's a marine block - but most of those won't have the side oiling passages drilled.

All we know so far is that it's a 427 - cool - and it was made after 1966 when the bellhousing core mold came from, and before the move to the new foundry line. If it's drilled for hyraulic lifter oiling it's probably 1968 or later...