The wide base allows for more range when bolting to the roof rafter and the 3 points of attachment for the arm allows you to move the arm according to the location of the barrel on curved profile models.
Batten bridge has a 9mm x 40mm slot and is 86.2mm wide. 10mm x 38mm rail slot accepts 5/16-inch or 3/8-inch bolts. Use the 5/16-inch Solar Screws in the 9mm slots.