Hi Gentlemen,
Tore, great spotting. I like your theory very much.
Don, according to
US Navy Report on type XXI U-Boat, this vessel was equipped with:
two Junkers compressors located in the aft part of the diesel engine room;
one electric compressor located in the pump room under the control room.
The "exhaust valve" visible on the photos provided by Tore is located in the aft room, on stb side:
https://www.flickr.com/photos/clemensv/9447842743/I don't think that Germans led the exhaust duct from diesel engine room, through electric motor room, to the aft room.
I have attached photo of the original bold ejector installed in aft room (at stb side) of U 2518.
I think that post-war version of the ejector (as Tore suggested, of NATO standards) was installed in place of it.
It could be used as bold ejector as well as signaling flares ejector.
The question is, why Norwegians did not remove the Junkers compressor, when they decided to put the bold/flares ejector in place of the exhaust valve?
Generally, such post-war ejectors were called (at least in RN)
Submerged Signal Ejectors.
Here you can find description of three types of ejectors: MK 2, MK 4 and MK 7:
https://maritime.org/doc/oberon/weapons/part2.htmIn my opinion, S.S.E. MK 4 is slightly similar to the arrangement found in aft room of U995 and U 2540.
At the bottom of the page, you will find some drawings of the "items" ejected from this device.
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Regards
Maciek