MGI Tactical? What part is the "receiver?"

[bkb0000]

https://www.mainemilitary.com/productcart/pc/viewcategories.asp?idCategory=212&pageStyle=H

http://www.gunbroker.com/Auction/ViewItem.asp?Item=128466996#PIC

i dig innovation, but i'm not sure what to make of this stuff

[Telperion]

The rear half that has the serial number is the receiver.

[Puffy93]

The rear half is registered.

It's a good idea but it cost way to much.