The yoke clamp base will fit rails in diameter from 0.625" to a maximum of 1.25". The 1" rubber ball is at a right angle to the plane of the mount; when connected to a rail, the ball sits parallel to the rail.
Material: Powder Coated Marine Grade Aluminium
Ball: B size 1" rubber
The "U" in the part number reflects product packaged in poly bag. The rail does not need to be perfectly round to mount this base.