Rails
The rails are 64" long and 11" high. Each rail is secured to the hull at the bottom with a screw horizontally positioned through holes it. There is wood inside the hull at this mounting point, which is not the case undermeath the rail terminus.
Center Support
The vertical piece of tubing (6" long) is supported by a base fitting attached to the wooden side rail. The base fitting is secured to the tubing by an internal plug with a machine screw. The base measures 15/16" by 2 7/16", with screw holes 1 11/16" center to center. The existing T fittings can't be removed, due to the bends in the rail tubing. They are attached with machine screws to tapped holes in the tubing.
Standoff Fittings
Each rail is held in position with two standoff fittings, secured with machine screws to through-hull mounting brackets, which are cast metal, incorporating a backup plate. The fastening points are 1 1/4" center to center. The mounting brackets accommodate a hull thickness up to two inches. My standoffs position the rail 1 1/2" from the gunwale in the bow and 1 1/8" in the stern. For the stern, 1 1/4" would be better for my desired rail location.