The 6″ Silicone Membrane stick is shipped with the end plug out of the hole to fit in the box.
When it is time to assemble put the plug in the end of the tube that is the deepest. The short end is where to hook up the air line. Simply look into the tube and you will see a short distance of a longer distance. The short distance is for the air inlet and it is 1/2 national pipe thread.