The Friction Knob UMS allows mounting plates with a ¼”x 20 mounting threaded stud to be connected directly to the end of the arm.
If you have difficulty connecting a mounting plate, it’s likely the mounting arm sections are backwards. The arm section with the reinforcing hex screw is the section that should be inserted into the Super Clamp. The opposite arm section should have the ¼”x 20 mounting thread to allow you to connect your mounting plate correctly.