So on the end of the wheel facing forwards in the picture, with the hollowed-out inside, you can fit the top of a stud into, and on the inverse side you can fit the bottom of a stud into.
Simply use this hollowed-out stud in place of the normal stud, and you can fit the modern towball piece into it for a quick and easy (if slightly fragile) leg design.
Best of luck in your endeavors.