ByJohn S. McClenahen Affected by change in the U.S. money supply, the Conference Board's index of leading economic indicators slipped 0.2% in September, ending four consecutives months of increases, the New York-based business research group reported Oct. 20. The leading index now stands at 113.0 (1996=100). Economists had been expecting the leading index to hold its own -- or to drop only 0.1%. The index of leading indicators, nevertheless, is up by 2.3% from its recent low in March of this year, notes the Conference Board. And, in another positive sign, the group's index of coincident economic indicators advanced 0.1% in September. "The upturn in the leading index since March has already been followed by a pickup in economic growth (both real GDP and the coincident index) and other signs are emerging that moderately strong growth is persisting for now," says the Conference Board. "While it is not likely that September's small decline indicates that the recent upward trend in the leading index has ended, a continuation of stronger economic growth would be called into doubt if the leading index does not turn up again."