I think that this is a filter rather than a rebreather, like the Proto in the photo from TheGreatParis. In a rebreather, the same nitrogen is used over and over, after the CO2 is removed and oxygen is added. This allows operation in an atmosphere deficient of oxygen or with gasses unanticipated when the filter was designed. This mask OTOH, seems to have only one hose from the filter and an exhale valve on the other side of the mask.
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