The final edition of The Post Aug. 21 carried a Reuter news agency report that Philippine opposition leader Benigno Aquino Jr. had been arrested but otherwise unharmed on his return to Manila.

It being a Sunday, when most of us don't get to the newspaper as early as we do on weekdays, some of those 27,200 subscribers who receive the final edition had already heard via radio that Sen. Aquino had been assassinated. The story noted that gunfire was heard and that a man had been shot, but quoted the airport manager as saying, "It was not Aquino. He was brought out. I don't know where he was taken."

To those who requested clarification:

The foreign desk, which was holding space for news of Sen. Aquino's arrival, received the story at 2 a.m. Shortly thereafter came accounts from AP and UPI that, according to the desk, confused the picture. AP said there were conflicting reports that Aquino had been shot, while UPI was "more adamant" that he had been killed.

With the last deadline pressing, the desk had decided to use the earliest but most conservative Reuter account with the headline "Aquino Returns to Manila, Is Arrested." The desk sought to add a sub-headline--"Shots Fired as Aquino Returns . . ."--but was informed it was too late.

The desk's dilemma is evident in a note prepared for Foreign Editor Karen DeYoung: ". . . almost would rather have had the page killed than (only) say . . . Aquino was arrested when he could turn up in the morgue . . . . On the other hand . . . was not prepared to say he was dead based on the evidence presented by the other(s). . . . The Reuter account made it clear that it certainly wasn't clear who was in the pool of blood at the foot of the tarmac."

Reuter did not report that Sen. Aquino had been killed until an hour and 25 minutes later. Margaret Klein, the agency's managing editor, explains: the 2 a.m. story was justified in view of obvious confusion surrounding the plane's arrival, where Sen. Aquino was being led by security police (who had come aboard) and because it quoted a "legitimate official"--the airport manager. A new lead filed at 3:15 a.m. still did not report the assassination. Ten minutes later, another story quoted a West German passenger and eyewitness as saying that Sen. Aquino had been killed. Subsequent reports identified American sources providing confirmation, although Margaret Klein says official confirmation from Manila police was not obtained until four and a half hours after the shooting.

Erring on the side of caution is not news business tradition. In cases like this, editors shudder at the alternative risk of reporting an assassination that did not occur. There have been infamous instances in the past where the news media have gone wrong both ways. Without its own correspondent in Manila and having to rely on the agencies, The Post decision in this case can be justified.

Some other loose ends:

* In late July, The Post ran a piece from Knight-Ridder by James McCartney reporting an administration "shakeup" involving Assistant Secretary of State Nicholas A. Veliotes, saying he would be reassigned and "the move . . . will probably be greeted with enthusiasm in Israel where Veliotes has been considered to be sympathetic to the Arab cause." With no evidence or attribution to document it, the remark, I felt, had no place in the story. Another newspaperman called it a "smear."

Mr. McCartney acknowledges it would have been "wise to have backed it up with more amplification." He maintained, based on conversations with certain Israelis, "what I wrote is valid," noting that Veliotes was in the job when President Reagan put forward his Mideast proposals that Israel has denounced. This, it seems to me, is still insufficient basis for an unattributed editorial judgment in an otherwise straightforward news story.

* A front-page story Aug. 16 noted that during the Reagan presidential campaign, William J. Casey received a memorandum that conveyed "a strong unavoidable inference" that "someone working for Carter" was "supplying information to the Reagan campaign." Written by Bob Woodward, the story quoted an unidentified FBI investigator saying the memorandum came from Max Hugel and was found in the Reagan pre-election files. Mr. Hugel, who served briefly in a senior position at the CIA when Casey became director, resigned after questions were raised about prior business affairs.

Washington Times reporter Jeremiah O'Leary subsequently wrote that unnamed sources "confirm" The Post account. Yet, evidence is hard to come by. The Justice Department has told the chairman of the House subcommittee on human resources, Rep. Donald Albosta (D-Mich.), who is investigating how published Carter papers came into the possession of Reagan aides, that it has no knowledge of the alleged memo. That, plus an FBI "no comment," were noted briefly in a Post story Aug. 25.

Meanwhile, counsel to the subcommittee James Hamilton, the subject of that story, says he will continue to pursue the report. Post editors say the same.