I have an '82 240D and I have seen a valve body by the battery as you described. In my case, if I remember correctly, this valve body has three openings. There are two valves (and corresponding two openings for each valve)that are mounted on the ends of the valve body and are connected to control cables that lead to the inside of the car. I have been able to trace these two individual valves to the two big rotary knobs inside the car. I believe they are to control the amount of heated water traveling through the heater core....correct me if I'm wrong.
The third opening on this valve body leads away from the valve body to the front of the engine near the water pump.
Anyway, is this the famous monovalve because what you describe seems to be something different from what I have. Do these valves, like mine, give alot of trouble? I have been operating the heater with no trouble whatsoever. I live in Houston, Texas where it has been getting chilly (around 35-40 at night) and the heater performs exceptionally well.