A 1st Knuckle adaptor is for old 15mm square hole, vertical style hitches. The 1st Knuckle adaptor makes it possible to use an older Adam's Trail-A-Bike with the new and improved hitch. All Trail-A-Bike models from 1998 (or one with a 15mm vertical lock pin) require a retrofit to replace the old knuckle with a 1st Knuckle.