Was the cab swapped out at some point?
Did you check the top of the frame, drivers side above the front axle? It may take a wire brush to clean off the dirt and grime to see it...
The serial number format changed between 1965 and 1966...'65 has the year "5" as the first number in the serial number sequence and '66 has the year "6" as the 5th or 6th digit in the middle of the serial number sequence just before the letter for the assembly plant.
Mike B
