My best guess for that rust is there is a gap there when the body is fitted and years of crap sit there get wet and that starts it off. Chassis not rusted because of the timber running along it.
Something is holding the plate in place and I would be looking at the vertical part of the chassis rail. Also looks like there is still a rivet not fully removed in the front hole. Need a pic of what is inside the C section.
Cheers Cobba & Cobbarette
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