Sorry for the delay getting back to you...it was finals week!
It is a new Bushmaster upper so as far as reciever holes go I guess it is the opposit of the old SP-1 upper; I'm not sure which is which, if you know tell me or I will ask my gunsmith.
As for the barrel, it is a Milspec pre-ban 20 inch. The CavComp was "permanently" attached, to make it post-ban legal, but it can be removed if desired; the threads were not destroyed during installation.
I have decided to reduce the price to $450.