According to my refrence data (located here on the Stovebolt Pages TechTips section) Chevrolet Serial numbers provided the "month" data from 1932 thru 1953.
As Jeff mentions in the reply above, it's the final letter before the production number that indicates the month in serial numbers from '32 thru '53.
What that means, is if you enter a serial/vin number for a truck made BEFORE 1932, or AFTER 1953, the "Month" portion of the data will say "unknown" since the month of mfg cannot be determined with the serial/vin number.
