It is part of your Swivel seat. there are 8 per seat. one pair of these is in each corner of the Swibel base. When the seat is locked forward they 'inter lock' to stop the seat wobble. Each 'Diabolo' is held to either the Top Swivel plate (rotates with the seat), or bottom swivel plate (fixed to the seatbase) with a small countersunk Allen head bolt through the Diabolo. Look between the two swivel plates and you will see the other 7 (hopefully).