heat shield linning

I think it's asbestos material.

 

There's conflicting opinion as to whether it's fiberglass batting or asbestos, but it's nasty stuff either way. Someone here had theirs tested on a later model, and it came back as fiberglass, but I recall some early literature describing asbestos. Since no one can see it, I'd think about modern heat shield wrapping if you're going to put the aluminum shields back.