Just went out and measured our baseplate. 5/8" gap between the tabs, 3/8" tab thicknesses, 24-1/4" center to center on ours. We have one of the older Blue Ox towbars, the kind that forms a triangle when attached, and has sliding, locking collars.
Hope this helps. I understand most of the newer Blue Ox are 18" on centers for the baseplates.