It's a pattern for the cloth cover to go over a mitered corner of standard FRC bumpers. The left side goes on a fold. The other heavy black lines are cut lines. The length will vary based on the length of a side; the dashed line indicates the end of the wood. The stitching goes along the gray line. I posted in response to post#2 in
this thread. The two lengths are given since there is no scale on an image.