If the diaphragm is ONLY on the rear carb, it's probably not what you're looking for. It is what the American MFG's called a dashpot to let the idle speed down slowly.
Keep studying the carburetor, there is something somewhere that is actuated somehow by vacuam and richens when there is low vacuam, and leans when there is high vacuam.
The Zeniths are somewhat complex because they have a primary and secondary on each carburetor that work like the primaries and secondaries on an American four barrel.
Keep on searchin' and don't give up, you'll find it.