This is a 90A Unit, Whereas the 13539 is 80A. Otherwise They Appear to Be Identical. Both Use a 6-Groove Serpentine Pulley. The 13507 Appears to Be the Same in a 5-Groove Pulley. Pulleys Should Interchange, as Both Have 17mm Shafts. In 11/2001, the 13683 Was Superseded to the 13538, as They Were Determined to Be the Same Unit.