This mount is designed to help center the mounting block onto the rail.  With a channel milled down the center, simply set the block on your rail, level it then drill your holes for the screws.  This block also gives a much more stable mount for your rod holder and your standard Jon Boat gunnel rail.
 
Made of 6061 Aluminum, not cast.
Milled channel for alignment and stability
Available in 3/8", 1/2", and 1/2" Left Hand thread
Comes with two(2) #12 stainless wood screws, 1 1/4" long.