If the buyer purchases older weapons in private sales that antedate the law and pays cash (or barters), then no one can prove the date of the transaction. Those which were initially purchased before the 1968 GCA (no papers) are often sought, at a premium.
Those 4473 are not supposed to be used to form a gun registry, but likely they have, even in violation of the law. Such was done with NICS data under Clinton, and the BATF got caught.
I doubt that hindered later totalitarians when it came to salvaging or recreating that database.
