This could just as easily be yet another government orchestrated "sting" to catch people with "machine guns" for media consumption and a great story about how the ATF is making America safer through their masterful diligence.
Or could have been bound for the cartels in Mexico, courtesy of the alphabet boiz, but somebody printing stickers in the carrier's shipment office got distracted.
In any case, those lowers should have been swapped within 8 hours of receiving and the FA lowers and parts destroyed.