I've looked at both '39 and '40 models, with matching patina original wheels, that had either, and I have a '41 with Schwinn script, so I'm guessing late 1939 early 1940??? Remember with Schwinn trying to pump out bikes as fast as possible there are no exact rules. Some special label brands like Chicago Cycle - LaSalle or Louisville Supply - American Flyer may have specified a specific hub for their orders. Also the custom shop that built specific brands and special orders may have used either depending upon the request.