From 1994 to 2004 the USA had a national “Assault Weapons Ban”. Firearms and magazines made before September 13, 1994 (the day the law went into effect) were exempt from the law. These grandfathered items are commonly know as “pre-ban” firearms and magazines.
These firearms are exempt from the “two feature” test which restricts what (primarily cosmetic) features can be included on the firearm:
Detachable magazine
Telescoping or Folding Stock
Flash Hider or Threaded Barrel
Barrel Shroud
Pistol Grip
Bayonet Mount
Grenade Launcher
etc
The federal law expired in 2004. Meanwhile in 1998, the Massachusetts legislature voted to keep the assault weapons ban in effect permanently in the state.
After the 2016 Massachusetts Attorney General “re-interpretation” the only way to purchase a complete AR or AK from a dealer is though a Pre-Ban Firearm.