Thank you. I agree with what you say, with additions: The J tracks have two positions to hold the sides (called flare boards in the AEDB record - I've no idea whether this indicates they are always flared or if they get up to vertical) horizontal and at an intermediate position before reaching vertical (or nearly so). The support rods look like they are probably round (makes sense as they would be equally likely to buckle in any direction under an axial load) I have no idea whether tube or solid bar. Tube would be lighter for the same strength, but weight never seems to have been a worry in CMP construction. There appear to be flat sections at the end of the supports for pivots to pass through. On the remains of the original body work on my trailer, the lower pivots are threaded and drilled for a cotter pin (the sides must have been removed a long time ago as all of the holes are clear - no rusted remains of cotter pins in any hole). In your photos, the upper ends of the rod appear to have a cross pin (not sure if bent or welded on) that must be threaded as there look to be wing nuts to keep it in place in the J slots.
That's about all I can draw from the pictures, I may have to make a mock-up to try to figure out the geometry of the bars and J slots. On first appearances, I'm not sure whether a straight bar with the cross pivots on its centerline would clear the reinforcements at the join between body and flare board if the flare board is truly vertical - another reason to make tha mock-up.
The pictures in the manual for the trailer are much lower quality than what you provided but they do all seem to generally agree.
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