Eh, pictures or it didn't happen.
Resistance after 5" means nothing really. Personally, I've never heard of such a thing. Doesn't mean someone didn't repair your care at some point with a rocker from a convertible. I doubt very much that it's intentional from the factory, but that said, stranger things have happened. Just because it was galvanized also means nothing, all of the metal was at some point I believe.
You could for sake of curiosity put an inspection camera into the cavity and have a look see.
I still maintain, you more than likely do not have that piece inside your rocker. Again, pictures will show reality. Actually, to the possible odd repair, I'd check both sides to be sure.