Here is an image of a 47-55 running board. A picture is worth a thousand words/posts: the square hole (one square hole) is shown (the hole referred-to by Cletis and the original poster). The other three round holes are on the top edge/lip. Jim Carter sells 6 rubber plugs for filling those round bed-to-fender apron holes that is unneeded on a panel-body truck.
My pickup is apart - is the square hole also used with the bed apron?
To answer the original poster, the square hole is not under the door opening, so it would not be used for a step plate. The step plate requires drilling holes nearer the forward surface of the running board (the reproductions step plates that I have bought do not have a hole-drilling pattern/template).
