Wrong. NW never took delivery of a factory-delivered DC-9; about half the current fleet was acquired from RC (and most of RC's DC-9 were acquired second-hand, discounting North Central and Southern) and the other half was post-merger via the mid-1990s.
I consider NW DC-9 as FACTORY DELIVERED even though its through a merger.
Other than the majority of the -50s, which delivered to North Central/Republic, virtually all of RC's DC-9 were acquired second-hand. Nonetheless, it's not being 'nit picky' since RC was a totally different airline than NW and the latter would've never purchased a DC-9.
I am not sure, if Finnair has already phased out the last -51. Also, I don't know if and how many VC-9s are still in service with the US Air Force and Navy. However, two operatos of original DC-9s come into my mind: Aeromexico has still some of its younger series -31 and Aeropostal of Venezuela still operates the original -51s. Any more?