The Super Apex fares to Asia seem to be the cheapest fares you can find on Northwest, but the fare basis code varies depending on the city you are departing from. For example, from Chicago O'Hare to BKK the best fare is in V class, a fare code of VLARC. However, from Milwaukee the best fare is in M class, a fare code of MJXAB7. Both of these fares are the "Super Apex Fares."

I suggest looking at www.travelocity.com. On their home page there is a place to list originating and destination cities, and it will find the best fares in that market along with the fare basis codes. To the best of my knowledge, all published fares you would find here to Asia on Northwest are upgradeable.