Northwest and Delta seem to have a policy of not re-registering their planes when they are acquired secondhand. I understand this saves some money, and it has the side effect of detailing the history of an airplane regardless of it's paint scheme.
NW has N and NC from North Central, RW from Hughes Airwest, RC from Republic, S from Southern, E and EA from Eastern, and L from Delta. They have been using NW for new aircraft in recent years, but have also used NA, and NB on the A319's. I also noted one of the newest A320's has tail number N375NC, which revives the North Central registration
almost 20 years after the merger. Evidently
375NW and NA were unavailable. NW planes used the US reg for many years since NW was a US flag carrier to the Orient.
Delta boasts planes with W and WA regs for Western, and WP for their ex-Western Pacific 733's.