It is certainly possible that some might be setup for less than 15 inches bu the vast majority of equipment is set up for 18 inches vertical height on category 1 implements. If you measure from the bottom of the hooks on the lower quick hitch to the bottom of the top hook it should measure 15 3/32 inches. You can lay a straight edge in the lower hooks and measure this easily enough. The extra 3/32" is the difference between the pin diameters but this still works out to 15 inches center to center.
