The front bumper is attached to the bumper re-enforcement plate in the front.
the rear bumper gets carriage bolts.
4 carriage bolts attach the re-enforcement plate to the 2 side bumper kick downs that go to the chassis.
in the front you have those Philips head bolts, the washer goes on the outside against the chrome or rubber bumper face, bolt goes through the washer/bumper into the re-enforcement plate with the C nut slid into the plate to lock everything together.
the real difference is the chrome bumper got the Steel washers, and the rubber bumper got plastic or Teflon washers.