Simon.
Very interesting to see how it is done, it`s a big jigsawpuzzle. Seeing you drawing I assume the black rectangle toward the pressurehull is the airduct from the main engineroom airintake valve. To me it looks as if it is one frame too far forward, see my drawing below. The reason for such a duct was that in bad weather seawater filled the airduct via the main intakevalve in the towercasing and water gushed into the engineroom, you didn`t want that water into the ventilation system so it was lead down into the bilge.
Tore