I have them under my bench in the '75 Dart More Door.
1/4" thick plastic.
Car is a column shift and it looks like without them, the seat would be down pretty tight on the center hump.
Only had the car for a Year so I do not know if the were factory but when I removed the seat, they were pretty well stuck down into the carpet mat / insulation.
They were installed as needed by the dealers; i.e. if a buyer was short and complained the seat was too low, they would install the spacers. If not, no spacers. So use them or not to suit yourself.