I recently purchased a copy of the IC Rules for Receipt, Handling,
Stowing, Bracing and Delivery of Less then Carload Freight Issued in Feb
38 and revised in Oct of 49. There are two illustrations of methods of
blocking car doors open to provide for ventilation one for a wood door
car and one for a steel door car.

I am guessing that they were hauling livestock like crated poultry. I
never heard of using a closed car for this before.