The top of the decontamination system is on the left with the spring loaded diverter valve. The return air intake is the screened pipe to the left of it, below it's own diverter valve. The pipe coming into that valve from the left is the fresh air intake, so that diverter determined whether incoming air came from outside or from the airlock.The springs hanging from the valve handles would have had cables attached which would run down to the switch that activated this part of the system, operating both valves at the same time.

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