Don.
The plan you are referring to is the LP system,practical identical to the IIVC system. As you know there are two ways of adjusting the trimsystem, normally by the trimpump or by LP air. Using the trimpump implies operating a closed system eg. the air and water volume is constant. When you pump the water to the aft tank the equivalent volume of air is replenishing the watervolume in the forward tank via the LP blow/vent pipe. In that case the LP air pipe is used as an "airshifting" pipe and the enclosed air is free flowing through a central bore of the aircock whereas the LP air connection is shut, you add nothing to the system.
If the trimming is carried out by LP air, you add air to the system as you turn the trim cock to put the required trimtank under pressure and simultaneously the other tank has to get rid of the excessive air to accommodate the receiving water and is vented via the cock and funnel to the bilge. The water is controlled by the trim valve and the water meter as normal. As you normally don`t want to change the amount of water used for trimming, you check the air cock for possible water in the funnel indicating when the receiving tank is full.
As you see, the central bore for the air cock is not drawn neither on the plan for the VIICs or IXCs, on the trimlineplan for the VIICs however you see the bore clearly marked, I assume you`ll find same on the trimline plan for the IXCs.
Tore