The Roadmaster Direct-Connect Base Plate Kit - Removable Arms Item # RM-521440-5 and Roadmaster Crossbar-Style Base Plate Kit - Removable Arms Item # RM-521440-4 are the same base plate but with different attachment arms. Since you're using a Roadmaster BlackHawk 2 All Terrain, Non-Binding Tow Bar - Motorhome Mount - 2" Hitch - 10K Item # RM-422 you can use either base plate with no adapters needed. view full answer...