[bcampbell]

Here is a cross section of one of the mounting points. The bumper is on the left and the U Channel is recessed into a pocket on the wood of the bumper. This allows a tight fit between the outside of the frame and wood part of the bumper, the top of the 1x1 and the U, and the outside part of the 1x1 and the outside part of the U. The pin goes all the way through both sides of the U and the 1x1 with a .25" hole.

To take the bumpers off, slide the pins out and pull up vertically on the bumpers. To put them on push the bumpers down until the brackets sit flush on the 1x1 and push the pins in.