sebosfato,
that isn't a tube in the center under the plate,it's a piece of stainless bar stock that is used for the negative wire hook up.It has a hole drilled and tapped in the center.There was an "O" ring that went between it and the bottom of the cell,to seal the bolt that went out the bottom.I took it apart because it was leaking,so I replaced the "O" ring.Thats why I took it apart,just took pics while I was at it.
The color of the tubes is from the brown scum that comes out of the water when runing it.Over time it discolored the tubes.It does come off if you rub it.
Don