On mine the plywood was on the inside,but that was just homebrew,not specially manufactured.That truck was parked at the end of the war when they could buy something better.I kind of forgot about the factory made side curtains,They had louvers,but they could be shut by hand.I'm not sure if they went on the inside or outside.They were made by one of the winterfront manufacturers,like Pines or Perfex.They bolted on with 4 or 6 bolts.