Thats almost true, but not quite. There WAS an amnesty period in the 60s' where they could be Grandfathered in. And, some of those guns do not have a physicial stamp but are registered in the great gun book.......ATF can tell you, BUT if it aint, ya got a problem. It can be surrendered, or under certain circumstances donated to a museum or such like, but you can't keep it. To further confuse the issue, some variations were exempted....