The LJ was the most popular Mack of its class. The SW in the model indicates six wheels (tandem rear axle, 6x4) and the X is for severe duty. The spotting features of an X suffix model are the exposed cast radiator and the running board / fender design. Built from 1940 to 1956 this truck was popular with contractors and heavy haul truckers. Because they were tough trucks many are seen today at truck shows.