A securities industry trade group's initial computer tests last week for year 2000 glitches were successful. The Securities Industry Assn.'s test turned ahead computer clocks at 29 brokerage firms and 13 exchanges and clearing and settlement firms, then ran simulated trades to see if they would work. Based on preliminary feedback, the trades were successful. More demanding tests are to follow. The tests, begun on July 13, were designed to pinpoint signs of the so-called millennium bug, in which computers could be unable to read and interpret dates after Dec. 31, 1999. In the SIA test, a four-day trading and settlement cycle was simulated, with mock trades executed on Dec. 29, 1999, through Jan. 3, 2000. Testers included Merrill Lynch & Co., Fidelity Investments, the NYSE and Nasdaq.