To protect all the springs, upholstery and padding and to give the seat back greater strength, cardboard panels are mounted on the seat
When you go to change the upholstery of the seats, you will find that these cardboard panels have had better times, so it is rare that these panels can still be reused. If you don't replace them in the front seats or don't fit them at all, you will always see it later, the upholstery will collapse on the back after a while because there is not the right support.
Originally, the seat panels and rear seat panels were made of pressed cardboard. In the reproductions mentioned here, those of the front seats are the same as the original, so they are also made of pressed cardboard.
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