My 473 has three separate tanks, and therefore 3 separate valve handles near the water pump. You mentioned the boat is new to you, so I suspected that your haven't located those valves yet. Mine ('02 Bene) are small finger valves each about the size of a silver dollar. They are underneath one of my settee cushions just forward of the galley (right next to the water pump).
Additionally, I see nooooo reason why over filling would do any damage to the tank (unless you were using a fire hydrant).