Until 1981 -- when the NHTSA standardized the VIN system to use 17 characters -- auto manufacturers we able to design and use their own serial number or VIN formatting. During the 1950s, Ford vehicles used a 12-digit serial number. On the 1955 F100, this number provided only the basic information. The first three characters indicate the truck style, the fourth character indicates the engine, the fifth indicates the model year, the sixth indicates the assembly plant where the truck was built, and the remaining six digits are the production run number.
In the Truck, Under the Truck
You'll find the VIN number on a stamped steel plate inside the glove box. If that plate is missing, the number is also stamped into the top of the passenger-side frame rail, just in front of the forward body mount.