Hy Packard Joe, my research this morning shows the first mention of 13" X 2.5" rear brake drums on K20 models as appearing in 1972, Bendix p/n 140363 there is no mention of single rear wheel or dual rear wheel, only one p/n. The first mention of single rear wheel/dual rear wheel I find is the 77-79 K30 w/13" X 2.5" rear brakes SRW p/n Bendix 140387 DRW p/n Bendix 140388. Are you sure this rear axle is original to your truck? If you could give us more dimensions, such as the overall height of the brake drum we may be able to find a drum by dimensions, hope that helps.
