Most Ensigns were built before HINs were mandated. Ensigns built before HINs have only a build number. For example, my boat is 513 which corresponds with the order in which she was built. To register a boat or change registration or in some cases renew a registrations, your state registration authority will assign a HIN. The assigned HIN has nothing to do with the manufacturer, order of build, month of build, and so on. It is just an arbitrary number that has the correct number of letters.
In South Carolina, if you were able to register the boat with the original build number, the boat can keep that number as its registration number. If the boat were to move out of state, however, the new state would issue a new HIN to be affixed to the hull. I hope this helps. By the way, the Ensign yearbook gives the years of manufacturer for each hull number. My 513 was built in 1965, I think -- I would need to look it up.