Here is my version of the BB smack Gen 4, I did not use the fill tube to refill the cells in the Gen stack. I just plugged up the center top hole instead.
I use the tube at the bottom to fill the cells there is a very small hole at the bottom of each plate to equalize the fluid level. It still has the sight windows on it. upper left and the slot vertical slot along the right side.
Glen
Mother Nature educates all of us that are teachable. She's hardest on the ones who refuse to learn. Punishment is automatic, immediate, and without pity.