Just out of curiosity, would the Mass Registry of Motor Vehicles have been involved in the title transfer? It is (was) after all still a car.
I realize that it was probably transferred as an artifact and not a street-legal vehicle, but if so there may be paperwork somewhere. Just wondering.
Government owned vehicles generally aren't registered with any MVA. In most cases the registration number on the side of the vehicle is also the tag number. Even id the tag number is different, it is still a US Gov tag on it that is used mainly for ID purposes. They also usually don't have a title.
Years ago I was riding on a Navy fire truck that was involved in an accident while on a mutual aid call with the local jurisdiction. The driver of the fire truck was charged with operating a vehilce on a state road that was not registered, because we had no registration card and only the USN tag and vehicle number on the side. That charge was later dropped but not before creating a administrative nightmare for the guy that was driving.