1984 300D ACC (Auto. climate control) heater core.

Anyone had one of these out or seen one that was out ?
Why is there one nipple on the driver side of the firewall and two on the passenger side ? Most vehicles of the past that I have worked on had a heater core with two openings...coolant goes in one side and out the other. I'm suspecting it has something to do with defrost/defog. Thanks.

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Diesel-guy