Moparlee, Because I get the correct measurement at station 1. Correct at 2,3,4 , then at the rear, (D) I get 18 on the rear. Unless I'm goofy. Where the drawing shows the rear leaf spring perch mount is way forward of where these are. These are close to the rear panel. At the rear of the rail where it attaches to the rear panel I get 18. The drawing shows the rails straight, horizontally and vertically. These are curved horizontally and angled rear to front. Now I know it doesn't matter what the drawing looks like as long as the measurements coincide with the positions of the measurements on the drawing. But according to the drawing the rear measurementis taken between the rear perch mount and the end of the rail. That's where I took it. Also took it at the rear of the rail. Same. And both sides are the same. I could understand it if it was a half inch off....but?