[steve v]

I'm not sure what book you're reading but it is wrong. Here's how I remove a '70-'72 A body rear bumper.

First disconnect the tail light plug ends and license plate lamp plug from inside the rear trunk area & push the rubber grommets thru to the outside of the car.

Leave those outer brackets on the car!!
There are 3 - 5/8" head bolts on each side that run vertically into these brackets, one lower one (shown in your pic) & 2 upper that you'll need a 12" extension to reach with your impact. After you've removed those take out the 2 bolts holding the center bracket to the body thru the license plate opening. Now you can slide the bumper straight back to remove.

Easy to do this way!