Not Ranked
I beleive the "N" or for that matter the very last letter in a ford part numbers indicates (I forget the term) when a part may change it's design from it's original design or style. That block was originally designed in 66 for a particular application but when it is carried over to another year only the date code changes not the part number. If small changes are made to the block over it's life than the last letter would change.
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