Maciek.
Sorry about my drawing which I made in a hurry mixing the pressurehull framing with the outer hull thanks for correcting and Don don't`make use of my drawing, use Macieks green correction.
The German text indicate that they had trouble with water even entering the Junkers exhaust pipe, it seems they already in 1941 tried to make modifications. Of course the stern buoyancy tank was an important element in keeping the exhaust outlet up to prevent intrusion of water in the exhaustpipe increasing the risk of waterstroke, upsetting the balancing and the two stroke scavenging as well.
I think we can make certain that the handle is the stern buoyancy tank venting handle converted from a conventional vent wheel to a 90 degrees turning handle with interlock preventing the Junker compressor to start ( exhaust valve to open) unless the stern buoyancy tank vent is shut, thereby keeping the stern up and exhaust pipe free from the seawater. I have made a sketch how I assume the interlock is working below.
Tore