a lower reciever for an ar-15 (or other gun)
that is made of polymer (or metal)
that is sold as 80% finished
so it does not need a serial number
you can then do the other 20% of the work yourself
then add the other parts (stock, barrel trigger etc) to finish
thus the comment
"everyone should have at least one untraceable polymer-80 ar-15 style rifle in their home"