Re: Nut on the downstem
I remember someone thinking that it was used to reinforce the metal of the stem, but for the most part, I don't believe there is a real 100% reason for that nut. Some of mine have a nut, others have a ball welded on the end.
I believe the diffuser from Hookah-Shisha will fit over this (ball or nut), if you are wondering. Or you could "make" one yourself.