This could be a workable option for the sides of a flatbed trailer(or possibly even one that already has short rails), specially if you have a good solid metal frame, or even not, it just wouldn't be as heavy duty. I believe these came off of an old 36 flatbed Ford farm truck. I recall seeing 2x4s with a slight angle cut at the bottom, and wedged into brackets like these with a hammer. If you used solid treated 2x4s it could certainly be sturdy, and easy to remove also. I looked at these, and sure enough, they aren't exactly square, but are tapered. In case you want em the measurements are (outside is 3" at top, 2 3/4" at bottom)(inside is 2'3/4" @ top, 2 1/2"@ bottom), height is 3"
I just noticed something else and checked the bolt measurements. These were likely bolted into wood, the bolts are all 1 1/2" inside measurements. And I'd guess that hole in the middle is to drive a nail through ; )
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