Help on a New Haven 485T

I have an old New Haven 485T that I am replacing the barrel on. I was able to cross reference the New Haven to a actually be a Mossberg 385KB, and obtained a Mossberg 385KB barrel.

Between the reciever and the barrel the is a metal piece that resembles a rear sight. The replacement barrel I recieved screws in into the reciever, but does not have the "rear sight". I cannot tell if the "rear sight" is attached to the reciever or the barrel. I just want to make sure I have the right barrel, and that the old barell will unscrew, before I tear something up. Any help would be appreciated.