Mounting bumpers with L angle

[smcmahon]

We have long rails running the across the perimeter of our frame at a height of 9 inches. We were considering using L angle attached to the back of the bumpers to sit on (and mount to) these rails to support and mount the bumpers. The L angle is 1/8" wall thickness and 1" square otherwise.

This sounds legal to us per R-21-F, but there's a small drawing (FIGURE 4-7) that's confusing us. It shows "hard parts" may not extend back more than 1". This is shown across the top of the bumper though, not where it may attach.

Am I misinterpreting the diagram in 4-7?