Actually, McDonnell already owned Douglas when the DC-9 Super 80 came out. Somewhat early on in production, they decided to call it an MD-80 instead. The MD-81, MD-82, and MD-83 are all officially registered as the DC-9-81, DC-9-82, and DC-9-83, but it is common to call them MD-s, since that's what MD marketing started calling them. The MD-87 and MD-88 were registered as MD-s.
I don't know what type of plane you'll be flying, but I think the timetable uses the code D9S for the DC-9-30 and D95 for the DC-9-50. I don't have one right now so I can't check it. You might want to call NW and ask them which type of DC-9 is on your flight.