I've been told that during most of World War II and for a period thereafter, the Office of Defense Transportation initiated measures effecting the national pool of freight cars. To obtain maximum service from a limited supply of equipment, the ODT required, among other things, pooling of refrigerator cars.
By ordering the pooling of reefers, particularly the RS types, the intent was reduce empty movement and improve fleet efficiency. The ODT order was implemented by Interstate Commerce Commission service orders.
Attempts were made to have roads east of Chicago assemble solid blocks of empty RS reefers of whatever western ownership and move these empties expeditiously to the West Coast for reloading.
This was not entirely successful because car types were not yet stenciled on all cars, and in yards choked with war traffic clerks couldn't afford the time to look up every car in the ORER.
Time was lost in Chicago switching out non-RS reefers from these blocks, and some non-RS cars even made it to California terminals before being turned back.
My question is, when did the stenciling of car types on freight cars become mandatory?