A former security guard said yesterday that a Quantico Marine who was convicted in the 1983 attempted murder, kidnaping and rape of a woman, was at a discount department store 12 miles from the military base when the woman was abducted.

A military prosecutor acknowledged later that the testimony could damage the government's case against Pvt. Lindsey Scott if a retrial is ordered.

The former guard, Cindy Taylor Ausby, now a licensed private investigator, said she became confused when she testified during Scott's October 1983 court-martial and failed to place him at a Zayre Department Store in Woodbridge the night of the crime because Scott's lawyer did not prepare her to testify or ask her to check store records.

Ausby's testimony came in the third and final day of a military fact-finding hearing to determine if Scott's original lawyer effectively defended him. It was the strongest point that Scott established during the hearings, which have attracted national attention.

"I can state emphatically that Lindsey Scott was in that store on the night of April 20, 1983, between the hours of 8:15 and 8:45," Ausby said.

Asked after the hearing if Ausby's testimony could be damaging to the government's case in a retrial, assistant military prosecutor Lt. Col. J.M. Messer said, "I'd be a fool to say that it's not."

Scott's current attorney, Gary Meyer of Alexandria, called the testimony "an absolute defense to the crime" and asked the military judge in the hearing to dismiss Scott's conviction.

The judge, Lt. Col. Hugh S. Atkins, refused, saying he had no power to do so. He also refused a motion by Meyer to let Scott, who is serving a sentence of 30 years' hard labor, out of prison.

The case now goes to a higher tribunal, the Navy-Marine Corps Court of Military Review, which will decide either to order a retrial, dismiss the charges against Scott or let stand the original verdict.

Scott's attorneys, from the well-known law firm of Howard & Howard, hammered away during the hearing at the defense that lawyer Ervan E. Kuhnke Jr. of Dumfries provided Scott at his court-martial.

They charged that Kuhnke had done little to contact or prepare defense witnesses in Scott's case, and had failed to pursue a number of possible defense strategies. Messer, the military prosecutor, said after the hearing that Scott's new attorneys had been "effective."

Kuhnke defended his performance in the court-martial, saying that he gave Scott the best defense he could. He also declared that Scott's conviction by a jury of seven military officers was "a miscarriage of justice."