Dennis I’m with you on the idea of the cars being lined with cardboard, ie for bulk loading of flour or some other fine fluid bulk commodity. Coopering for grain only involved boards covering visible voids in the car interior, ie the floor. But bulk loading of things like flour, sugar, cement, and other fine fluid commodities called for the entire car to be lined with paper or thin cardboard. Ground grain, ie feed stock, would qualify for the lining of a car.