PORTLAND (AP).- A treasure hunter said he has located the wreck of a British merchant ship that was torpedoed by a German U-boat off Cape Cod during World War II while carrying what he claims was a load of platinum bars now worth more than $3 billion. If the claim proves true, it could be one of the richest sunken treasures ever discovered. But an attorney for the British government expressed doubt the vessel was carrying platinum. And if it was, in fact, laden with precious metals, who owns the hoard could become a matter of international dispute. Treasure hunter Greg Brooks of Sub Sea Research in Gorham, Maine, announced that a wreck found sitting in 700 feet of water 50 miles offshore is that of the S.S. Port Nicholson, sunk in 1942. He said Wednesday that he and his crew identified it via the hull number using an underwater camera, and he hopes