The APWS System is the car's potable water. Almost every passenger car had one. The sysetm consisted of a tank or tanks to hold the water, an air tank to lift the water, fills for the tank and associated piping to get the water to the faucets and fountains. The system also included a way to heat water for the "Hot" faucet, but we do not need to condern ourselves with that because it was usually inside the tank. In truth you are looking at the tank cover. The actual tank, or often tanks, are inside. These parts are often original because they were made of stainless steel, the cover protected them, and the system hasn't been improved upon like air conditioners and retention toilets. So if you find a car at a museum, the APWS tank might well be the only original part under the car. They came in a wide array of sizes, but in the lightweight era most were slung under the center sill of the car.