I have a set of NOS step plates and the instructions for installation show the layout and drill requirements, there are no existing holes in the described location. The square hole is for the filler support brace that goes behind the filler panel.
https://www.flickr.com/photos/advance-design_parts_co/12616625763/in/set-72157631868452754
Here you can see the six attachment points and the holes that would be required to install them.
https://www.flickr.com/photos/advance-design_parts_co/12616923234/in/set-72157631868452754
6" From the rear edge of the door the is where the step is placed, the six holes are measured from that starting placement.