It was just a hole filler-bolt. That short bolt was supposedly non-threaded and had a spring-nut to hold it onto the bumper. The hole was vestigial - no longer used but formerly placed in the bumpers to attach a strengthening bracket for a hand crank (I have an old bumper with that bracket).
Jim Carter has a tech article about this (47-55, in Misc). Here is
a picture of the bolts. I think JC now sells a reproduction center (short) bolt.
