Until he took the stand today, a trim man in his 30s with a mustache and blue suit, he was a sort of "Mr. X" in the trial of alleged Marine turncoat Robert Garwood. The witness had been known only as the mysterious "European businessman" Garwood picked out in the darkness of a Hanoi hotel bar almost two years ago to deliver his cry for help.

This was at 10 p.m. on Feb. 1, 1979, and Ossi J. Rahkonen, a Finnish economist who works in the Transportation, Water and Telecommunications branch of the World Bank in Washington, D.C., was on assignment in Hanoi. He was having a drink with colleagues in the Thang Loi Hotel bar when a thin, unshaven, "shabby"-looking man approached his table and asked if he had any American cigarettes.

"No. i don't smoke cigarettes," he said, as the man surreptitiously pressed a scrap of paper into his left hand. Rahkonen read it.

"I am American in Vietnam," the man had written. "Are you interested?"

Yes, Rahkonen said, and asked the man for his name.

"Robert Russell Garwood," scrawled the American, and added his Marine Corps service number.

The note was the first message from Garwood in 14 years, except for a letter he had written to his mother soon after he was taken prisoner by the Viet Cong outside Da Nang Sept. 28, 1965, asking her to notify the Marines of his capture.

With that note, a bewildered, very nervous international economist suddenly was thrust into the role of a pseudo-spy who would help bring a controversial American prisoner of war in from the cold. Rahkonen's role in the return of Garwood, 34, the first American soldier to be brought to trial for misconduct as a POW in the Vietnam war, has been one of the many closely held secrets in the case.

In his testimony today, Rakhonen said Garwood "was dressed casually" and wore a cap "drawn over his face as much as possible."

"He said he had to deal with you quickly because security guards were watching, didn't he?" prodded defense attorney John Lowe. "He told you he wanted out of Vietnam, didn't he?"

"Yes," said Rahkonen.

Garwood asked the stranger to deliver his message to the U.S. State Department. He said he had been held in a forced labor camp 100 miles west of Hanoi and only had been able to slip out and travel to the hotel -- an official residence for foreign visitors interested in rebuilding Vietnam's war-torn economy -- because the nation was busy celebrating Tet. He was not free to leave Vietnam, he told the stranger, who saw him two nights later in another bar in the hotel, sitting with "two Caucasians." Their identities remain a mystery.

Rahkonen said Garwood said that if he didn't hear anything in three months, he would try to get word to some other neutral country.

After he met Garwood, Rahkonen flew to Washington and turned over the note to the State Department. Officials showed him photographs of Garwood, but he said he didn't recognize them because the pictures showed a younger, much heavier, healthier-looking man.

Rahkonen's testimony for the prosecution was among the last offered in evidence as the government winds up its case against the alleged collaborator.

But Rahkonen's testimony appeared to help the defense, raising doubts about the freedom that Marine prosecutors maintain Garwood had in Vietnam, and the possibility that other Americans -- the Caucasians" -- also might be being held against their will. The government of Vietnam claims that Garwood remained after the war as a free man.

The prosecutor was seeking to convince the jury of five Marine officers, all Vietnam veterans, that Garwood's presence in the hotel proved he was a deserter and a free man who stayed behind after other POWs were returned in 1973 and American troops pulled out two years later. The defense contends that Garwood was a prisoner, an emotionally unfit Marine who was tortured, brainwashed and driven into the arms of the enemy.

Garwood was last seen before his capture at the Da Nang USO canteen by a Marine who testified Monday that he overheard Garwood say he was on his way to make a laundry run, would stop at one of the dozens of "skivvy houses" or whorehouses, and would rendezvous later back at the barracks for a beer.