The panel was a Masonite product (wood shavings and saw dust with a bonding agent and pressure to for the sheet) shaped for the application
Texture of the panel
Between it and the package tray was a thin sheet of insulation - brown in color with a pressed waffle like pattern. Panel continues all the way up the back and is visible from the trunk side
Further forward
Forward edge of the panel is held down by a trim strip that runs just behind the rear seat upper section as visible in the picture below - the white stripe in the picture which has a end cap of sorts one at each end