You should call me!
That is the transfer port and it is a needed item. Assuming the transfer port gasket is still in place atop the valve exhaust opening in the tube, the smaller end of that transfer port goes down into the gasket. Then, with your barrel into the breech with its port aligned with the breech port (on the bottom) lock the barrel into place with the breech grub screws so it will hold still. NOW you can lower the breech/barrel assembly down onto the transfer port, then start the breech screw... check alignment... tighten the breech screw some more.
There is no difference in the way the aluminum breech (or any other metal breech) goes together... and is the same as the plastic breech (except for the grub screws holding your barrel in the breech).
Again, call me if you need a walk-through. 229 344-5355
David Grimes (Alchemy AirWerks)