the door jam tag has a serial number. Some states use that number others use engine number some use either. Both my 38 and the 54 are titled according to tags on the vehicle. To throw a another wrench into it my 38 truck was reengined in 1942 and retitled as a 42 even though it's still using the 1938 cowl tag number. Apparently at that time if you rebuilt or replaced the major components you could register current year sort of like glider kit trucks today.