Something else left out or forgotten by many is the ATF did NFA amnesties.
All illegaly held machineguns could be registered and become legal under an amnesty up until 1971.
That means guns held in violation of either the NFA of 1934 or the GCA of 1968 were still being legaly registered even up until 1971.
Prior to 1969 the military also allowed many trophies to be brought back from battle. Others illegaly brought some back even later, including select fire AKs and other weapons as late as the start of the Vietnam war that were then given the opportunity to be legaly registered under the ATF amnesties as late as 1971.
That means weapons illegaly possessed since before 1934, or aquired anytime since, including as late as the start of the Vietnam war, were legaly registered as late as 1971.
Whether legaly or illegaly purchased and then illegaly retained, illegaly aquired,
or illegaly manufactured, or illegaly converted from existing weaponry, they all were given the chance to be legaly registered during amnesties.
So there is weapons that would otherwise not match the legislation timeline that were legaly registered. Like foriegn full autos made between the 1968 GCA prohibition on importation of foriegn full autos, and the last 1971 amnesty.
Most similar programs in the world have been offered by a government to allow them to collect information on where items they do not like are being held. They enact harsh legislation for illegal possession, then allow people to escape the penalties through registration. Future legislation or actions at a later date then target those weapons or thier owners after the government feels the usefulness of the information gathering has approached its end.
As a result most people did not use the amnesties to register illegal weapons in the US, but some did.
Still the official estimates based on previously legal full auto sales, and war trophy bringbacks is that only a small fraction of illegaly held select fire weapons were registered.
The lack of success of the program from 1934-1971 or even until 1986 to register the majority of the weapons is probably why those who did register were not targeted later as normaly happens in the world.