If the flight number is "NW nnn" on your ticket, then NW fare codes apply, not DL. For DL flights, "U" is a discounted coach class code, and for NW flights, "V" is a discounted coach class code. Even more confusingly, it does not matter who actually operates the flight, what matters is who's flight number is on your ticket. If you really have a ticket on "NW nnn", then you should be able to view your flight on nwa.com and probably select your seat there as well, unless the flight is oversold and no seats are available.

This is one more piece of trivia that will not matter after inventory cutover (which is speculated on this site to be planned for 1/31 or 2/1 depending on who's postings on this site are right) after which every Delta flight will be located in one computer system and there won't be any more NW flights to confuse us all.