The 1968 GM letter specifying the CE blocks (and CT transmission) specifies that position in the sequence to be a progression letter to allow higher numbers... ie CEYrnnnnnn, then CEYrAnnnnn, then CEYrBnnnnn, as far thru the alphabet as required. It IS surprising that they had enough Warranty engine replacements to progress as far as 'N' thru the alphabet, but I believe at that time, GM had a 5 yr powertrain warranty...
Perhaps between '68 and '71 or '73, there was a further expansion or clarification of the warranty replacement parts?? If we can find such a clarification it might be explained further, but without that there are any number of conjectures that one might make to explain it, rather than go by the GM letter we do have...