Brendan - The first picture below is taken from this page in the
1929-41 Chevrolet Master Parts Book. Note that the 1940 Commercial 1/2 ton is identified as a "KC" model. The second and third pictures are from the
1940 Chevrolet Truck Data PDF, also confirming the KC Model year and decoding instructions.
The correct decode for your provided serial number is:
21 = Janesville Plant
KC = 1940 Model year Commercial 1/2 ton truck
1 11 = Built in the month of
January November 1940
12651 2651 =
11651st 1651st truck off the Janesville line for that year.
I'd say no doubt that the truck should in fact be a 1940 Chevy KC 1/2 ton, presuming the cab still sits on the original chassis, etc. etc...
Edit Note: After looking at later info posted by Brendan, I realized I incorrectly decoded the original info so corrections are noted above.
