The RAM-B-231U consists of a 1" rubber ball designed to sit at right angles to the mounting rail. The rust proof  metal u-bolt accommodates rails in diameter from 0.5" to 1" in diameter. Included are black cover caps to be placed over the exposed ends of the u-bolt and a composite adapter that can narrow the rail to a minimum width of 0.5". 

U-Bolt Dimensions:
Accommodates rails from 0.5" to 1" in diameter.

Material  
Powder Coated Marine Grade Aluminum 

Ball Size: 
1" Rubber Ball "B" Size 

Note  
The "U" in the part number reflects product packaged in poly bag.