I purchased a new in the box a Bachmann GE U 36 B diesel from an estate sale. It has what I would call an open coupler box ( no top) with the hook horn coupler attached by a brass step shoulder screw. I want to convert to the knuckle coupling but have no idea what parts are needed to accomplish this transition and the hole diameters are different between the respective couplers. Also, there is a step cast into the plastic sides of the open coupler box. Your help will be greatly appreciated.