I would guess that the hole at 2 o'clock is to supply air to the carbon canister as part of the evaporative emmisions control system.
This is how it is hooked in (assuming it is a 73.5):
© Dr. Ing. h.c. F. Porsche A.G.
1. Expansion Chamber
2. Vent Chamber
3. Activated charcoal container
4. Engine fan upper shrouding
5. Pressure line from fan to activated charcoal container
6. Purging line from activated charcoal container to engine air cleaner.
For a bit later car, the setup looks like this:
© Dr. Ing. h.c. F. Porsche A.G.
1 – Fuel tank
2 – Expansion chamber
3 – Activated charcoal filter
4 – Cooling fan upper shroud with hose connector
5 – Hose from cooling fan to activated charcoal filter
6 – Hose from activated charcoal filter to engine air filter
7 – Engine air filter
8 – Return hose connecting fuel tank with expansion chamber
thanks Grady! http://forums.pelicanparts.com/showthread.php?threadid=299053