I don't beleive that diaphragm in the front of the carb does anything. It doesn't appear to block or open any orifices that I can see. Nor does it seem to obstruct or restrict any orifices either.
The inlet fuel passage leads directly to the main jet fuel passage. The two are recessed so even if the diaphragm closes off the top the fuel still flows freely.
Remember they took a Saito 4 stroke carb and modified it to clone it. In the process they split the carb from the pump/regulator part. Thus the front of the carb was probably the cavity for the regulator or the pump. So they simply made a gasket to close it off. The fuel only flows through that small recessed Tee shaped hole over near the side. It does look sort of stylish with all the screws on it, but they could have simply had left that part of the casting off and just have a small part to block off over the top of that recess for the fuel flow.