Ares has them, too.
The white part is softer whereas the "receiver" part is kevlar reinforced. You use a slightly dull Dremel bit and just run it around in there. It'll cut the white part and not too much of the black part.
It is made to be the cheap and easy way. Consider it ultimately disposable, because it is not going to stand up to abuse like a 7k series aluminum forging will.
If you've got a drill press, half a brain, and a little bit of patience, you will be happier with an aluminum receiver, trust me. (People also finish 80% aluminums with a Dremel, I just don't recommend it as Plan A.)