Air-Filtration unit from a German bunker WWII

Many German bunkers from WWII had these anti-gas air filtration systems installed that had to manually be operated to pump outside air through an air filter in to the bunker. The round pressure-valves in the walls can be found in most bunkers. These are called "uberdruckventil" and that basically means over-pressure-valve and would automatically let air out in case of overpressure inside the bunker. Source: Objaverse 1.0 / Sketchfab