RE: Bob Nelitz 1/3 Scale J-3 Short Kit build
It's the firewall - and should be present on certainly most Cubs - however, not that often shiny - the one I'm modelling has had a restoration, and the guys decided to polish the aluminum up. Seen a few others like that too, plus of course a bunch painted black etc. The ribs are strengthening ribs, pressed into the firewall.
Hope that helps!
Mike