This is the piece your missing but don't really need, it a handy little soda shutoff that you just screw in the screw and it pinches the rubber insert and shuts off the flow.
Beware though! if you have the syrup tank full and you shut off the soda water the handle drops and your syrup starts to drain out.
This is due to the fact that the pressure holds the handle up, when you take it away the handle is so heavy it drops which allows the syrup to flow. Think about it, your standing there playing with your machine and all of a sudden your expensive syrup starts draining all over everything, easy fix just hold the handle up, problem is finding a clean container while your holding the handle.
One of my fondest memories is doing this exact thing at a S.S. Kresge's soda fountain, only difference is I had three handles to hold up and customers all around me at the bar to visit with.