Quote:
Originally Posted by Onkel Neal
..., and another set are located on the opposite (stern) end of the control room, they open the vents for the MBT #2. That's what I think.
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I think you thought is correct, stern of the control room is MBT#2(?)
(maybe someone can help to translate
)
"Handrad für die Entlüftung des Tauchbunkers 2"
Source: "Vom Original zum Modell: Uboottyp VIIC"; ISBN 3-7637-6002-4
Also take a look at 00:04:
(flood the MBT#4 ?)
Maybe this background can help too (it's not WWII but possible it was similar)
Definition for Bundesmarine submarines:
Source: "Unterseebootbau"; ISBN 3-7637-5958-1
Lenz-/Seewasser: Seawater 
Kraftstoff: Fuel
Frisch-/Waschwasser: (not salty) Drinkwater
Schmieröl: oil
Lüftung: air
Druckluft: compressed air
edit: a shape of valves is given for its purpose in case of darkness