Regarding the last of the 5 pictures, the two handwheels over the fore bulkhead of the control room, Buchheim writes on page 47 of his book "Die U-Boot Fahrer" that these are for the "
ventilation of the two saddle tanks in the boat's center". Since there are only two exterior (saddle) tanks at the center those can be either the control wheels for the two (aft) trim tanks, or for the two (fore) negative buoyancy tanks.
What does "ventilation" mean here, Bucheim writes "Entlueftung". Is this for letting pressure air into the tank for "ventilation"? Or is it for flooding, opening the upper valves?