On my feather duster the decklid was all steel and I could find no evidence of it being changed at some point. But I did notice that the side glass is thinner than the standard car glass. So that was also an area of weight savings.
My dads FD has a steel trunk lid as well. No weight savings at all there. Its only got 85k miles on it. Never been wrecked and I believe dad is the third owner. 100% factory.
