The Seattle-based aerospace company said the orders, placed by four airlines, are for the model 500 version of its successful 737 twin jet airliner. The $2.1-billion deal includes the 51 firm orders, and options for 22 more planes and spare parts, Boeing said. The 737-500 will have about the same passenger capacity as the older 737-200, but adds the advanced technology featured in the 737-300 and 737-400 series. Southwest Airlines of Dallas has ordered 20 of the 737-500s and a Norwegian airline, Braathens SAFE, ordered 25 of the planes. The charter airline Euralair of France has ordered three 737-500s and a fourth, unidentified carrier ordered three of the planes.