Hi Tim,
It looks like those are made by Restoration Designs.
I spoke with them and they explained those are meant to overlay the original longs and are slightly bigger.
The price is good as they sell them for $205 each plus tax and cost to bring them into the U.S.
Frame stiffener, left overlays longitudinal: Restoration Design
Frame stiffener, right overlays longitudinal: Restoration Design
Good luck with the sale.
Bruce