Japan's index of leading indicators--a measure of a nation's future economic activity--was 9% higher in September, the latest month for which figures are available, than the average of the preceding 12 months, the Conference Board said. Australia was next with a gain of 8% in the same month. The business-supported research group said the index for the United States, which as recently as April had shown an annual increase of 10%, fell to 1% by October. Other non-Communist industrial nations displaying promising growth potential included France at 6% and Canada and West Germany at 4%. Britain was the only major industrial nation to show a negative growth rate, down 1% in September.