I am puzzled about the reason for the notch also. If the notch is positioned toward the front, it will not clear the gas tank filler neck and you cannot pass the bolt through it. This is with an original tank and an original mounting block. True, I have a reproduction bed, (Mac Hils) but all of the other holes line up perfectly so I know that is not the problem.
I know of an original 36PU that has never been apart. I am checking with the owner to see how his is mounted. Oh Henry , where are you when we need help????? lol