Below: a 1939 advertisement for bomb-shelter doors. Schutzraum, “protection space,” is the German word for shelter.
This advertisement is from Luftschutzraum-Bauweisen (Air-Shelter Construction Methods”) by I. Frommhold. It is a booklet published by Verlag Gasschutz u. Luftschutz, “Gas-Defense and Air-Defense Publishing.”
Why use air-tight doors on an air-raid shelter? Prior to the Second World War, as the publisher’s name suggests, the possibility of gas attack was not considered out of the question. Also, in the event of fire, it might be advantageous to prevent all oxygen from being sucked out of the room, and hot air from coming in. Oxygen-deprivation was a major cause of death during Allied bombing raids.
For more information about these doors, including uses of the peep-hole, check out engineer Fritz Berg’s site.