The RAM-201LU, double socket long arm with locking knob, has a socket at either end to allow a 1.5" diameter ball accessories to be connected.

Overall Length: 5.625"
Socket-To-Socket Length: 4.5"

The "U" in the part number reflects product packaged in poly bag. For additional specifications, see components and drawings tabs.
Color:  Black Powder Coat 
Material:  Powder Coated Marine Grade Aluminum 
Ball Size:  1.5 Inch Rubber Ball