Found this too: -

"The coupling has a thick casting and to get the required 25 degree tilt on the ball extra clearance is required on the neck of the ball. The Alko hitch requires 65mm clearance below the centreline of the ball where the standard is 32mm. With a standard ball the Alko hitch will foul the neck of the ball when the trailer tilts sideways or forwards before the required 25 degrees is reached. "