A closed circuit rebreather emergency escape set known as a Momsen lung.

Invented by US Navy Submarine rescue pioneer Charles 'Swede' Momsen between 1929 and 1932.

Worn on the chest it provided breathable air on a slow descent to surface, the gas filled bag then doubled as a buoyancy aid.

Submariners trained extensively with the apparatus in 80-100 foot deep escape tanks in the U.S and it became standard equipment on U.S submarines. Post war analysis stated it carried inherent risks and it was phased out and replaced by the Steinke hood in 1962.

OSN is stamped in red capitals suggesting this was a Swedish variant from the East Coast Naval base Ostkustens Orlogsbas.