Simon.
Main drain valve.
From the amazing photos of U 534 is another component revealed, the details of the remote controlled main drain bottom valve. As can be seen from the photo below the valve is placed at the very bottom of the bilge, has as flanged suction pipestub and a rodconnection to the handle having a bevelgear connection to a shaft going through the bulkhead. The bevelgear is probably not applicable to the VIICs though.
Tore