Nick - The sequence numbers started at 1001 at each assembly plant.
Your tag with the "12" following the "KC" would indicate, following jimgmc's info, that your truck is a 1940 model and was built in December 1939. It should be noted though that your truck was built at the "Kansas City Missouri" plant as denoted by the "5" at the beginning of your data plate. Seems maybe the Kansas City Plant was cranking them out faster the Janesville Plant.
