Mark,
I do not know the material. Steel or aluminum would do fine. It is unpainted and unrusted metal. I do not think it is stainless steel, but that would work too. You would not want to paint it, because it would chip when the seats are removed/replaced (if they are/were replaced - that is why many seats are lost for/to posterity).
If I have not put that piece back in place (and, if I can easily find it), I take more photos and post them.
