Your best route is to get a Marti Report. Mine had the original dealers name and address, and the date of first sale.
That will give you the state in which to inquire. Registrations here are the jurisdiction of the individual states.Not the Federal Government.
On a car this old it's unlikely that there will be any record of the first registration. In 1973 only a few states were "title" states and just issued you a Registration along with the license plates.
There was not a uniform system of keeping the records from state to state. Even states like California with its huge quantity of vehicles basically dumped the records after a certain time. In some cases it's because they went to a more modern computer system but also there is not much reason to bear the cost of storing vehicle records that are 50 years old.