The original chassis serial number found on the cab A-post tag is also stamped on the frame. It's found on top of the drivers side frame rail right above the front axle and will have the Year, Plant and Unit number...the series code was not stamped on the frame...
Sample 56A001234
A 6400 will have a 154" wheel base and be coded "6B", so the cab tag would look like this...6B56001234.
The front fender emblems are 1956...
Mike B
