panel behind rear seat on '68 convertible

I don't recall a board of any kind in my 67 vert either but I didn't buy it new.
The notchback had a piece of flat black cardboard at the seat back. Not 1/4 thick though. Closer to 1/8. It served as a blind more than vapor/noise barrier or insulation. The verts boot well served the blind purpose. So did any vert come from factory with a board at the seat back? I doubt it.