Inside the aluminum doors of my XK140, there is card board like material (stiffer than cardboard) glued to the inside of the doors. I'm guessing it is there to stiffen the door skin since the aluminum is not all that strong. My question is this, was this put on at the factory or was it something a previous owner would have installed. If it is a stock item, does anyone know what it is and where I could find a like product to use when I restore the doors. Mac