In dueling advertisements, Northwest Airlines and its pilots union are trading shots over the value of stock the pilots' received in return for a pay cut that helped the company avert bankruptcy. The stock was part of nearly 900 million dollars in wage and benefit concessions negotiated in 1993 with the airline's largest unions. As a strike deadline tomorrow night approaches, both sides accuse the other using advertisements to mislead the the public over what the pilots got. Minnesota Public Radio's Bill Catlin has more. Ads placed by the Airline Pilots Association in today's major twin cities dailies refer to the concessions and a fifteen and a half percent pay cut the pilots accepted. It also says Northwest executives