The registration number is sort of like the military's permanent serial number/license plate number. It is not the same as a VIN number, or manufacturer's serial number. It tells you, in code, what size the truck was/and or its original use, sometimes when it was bought, and its sequence in delivery.
The FSN (Federal Stock Number) was the precursor to the NSN (National Stock Number) - it is the "part number" for the entire truck within the military supply system. The FSN can be used to identify the "M-number" of the truck, and some of the options that were installed on the truck when it was delivered.
HTH,
David Doyle