As we have seen any replacement block would be stamped "CE"
With this block being a late May 71 casting it could have been used in 1971 but with a September block stamp I think it was an attempt to be used in 1972 or restamped (nasty word) according to the original 1970 block stamp.
Again, CKO was not used in 1971 or 1972 and any block correctly replaced under warranty would have been stamped with the replacement code.
Rick H.