The difference between the Z2 and Z3 is the swivel hose and the purge cover bling. Nothing else. The monel piston is in the M1 and Ti series regs.
You are conflating two different features of the second stage. The Automatic Flow Control (AFC) eliminates the need to manually adjust the Venturi effect, which is what controls how air flows around the second stage housing of a regulator. Has nothing to do with the orifice.
As part of the assembly the orifice has a wave washer. When not pressurized the wave washer pushes the orifice away from the LP seat. As such, it is possible for water to enter through the inlet tube and into the hose. If the regulator is soaked such that the first stage is below second stage any air will escape and be replaced with water in the first stage. It is prevented by soaking while pressurized, which is usually not practical. Or by soaking each stage separately.