The recently released magistrate's report on the investigation into the shooting of Pope John Paul II has clarified an ambiguous phrase in the original prosecutor's report that led to an error in an article Oct. 17. Mehmet Ali Agca retracted allegations concerning a plot to assassinate Polish labor leader Lech Walesa on June 28, 1983, and not at the end of September 1983 as reported.

In late October 1982, Italian magistrates showed Mehmet Ali Agca a grainy black-and-white photo that seemed to provide crucial evidence that he had not been alone when he tried to murder Pope John Paul II.

The picture showed a slim young man, his back to the camera, running out of St. Peter's Square in Rome a few seconds after the assassination attempt on the Polish-born pontiff. The American tourist who took the picture, television executive Lowell Newton, had told investigators that the fleeing man had been carrying a gun.

Who, the magistrates wanted to find out from Agca, was this mysterious gunman?

Agca's answer, which was noted down by a court clerk, was stunning in its political implications for East-West relations. The running gunman, he told the magistrates, was an undercover Communist agent whom he had known by the code name "Sotir Kolev" but who in fact was Todor S. Aivazov, administrative officer at the Bulgarian Embassy in Rome.

It was not until some two months later that the Italian magistrates found out that Agca had lied about the identity of his apparent accomplice. The lie was discovered when Aivazov showed up at a news conference in the Bulgarian capital of Sofia on Dec. 17, 1982, for questioning by the international press. One look at his stocky, well-built profile made it apparent that he was not the gunman in the much-publicized photograph.

Some days after the press conference in Sofia, Agca changed his mind about the running man in St. Peter's Square. The figure in the photograph, he now told magistrates, was not Aivazov at all but a Turkish accomplice whose name he had wanted to protect since he was a "dear friend." A report filed in court by the Italian public prosecutor earlier this year says that Agca eventually identified this "friend" as Oral Celik, a fellow member of a right-wing Turkish extremist group known as the Gray Wolves.

The incident provides an illustration of how Agca repeatedly changed, updated and refined his testimony to both his Italian and Turkish interrogators. The overall effect of these changes was to bring his evidence into line with events occurring outside the top-security prison where he was being held as well as with revelations about the case in the mass media.

Italian defense lawyers representing the Bulgarian Embassy have already given notice that they will seek to exploit the changes and contradictions in Agca's testimony to undermine his credibility as a witness in any forthcoming trial. In preliminary submissions to the court, they have argued that details provided by the pope's would-be assassin about his three Bulgarian accomplices could have been "fed" to him either before or after he entered prison.

For his part, the Italian prosecutor has acknowledged that Agca "inserted untrue facts" into the proceedings. But he maintained that such mistakes were understandable if viewed in the light of various "psychological" pressures upon Agca and the efforts he was making to negotiate his escape or release from prison with "the only three possible bidders -- his Turkish accomplices, his Bulgarian accomplices and the Italian justice."

"Agca did this exactly like the Levantine that he is -- with elaborate overtures in presenting his goods, variations in price, color and weight. . . . By trickling out his first meager confessions, he was at first seeking the aid promised by his compatriots, then the help agreed with the Bulgarians, before finally entrusting himself to the clemency of our judicial order," prosecutor Antonio Albano wrote.

Insisting that Agca was coherent and convincing in his overall reconstruction of the crime, the prosecutor asked rhetorically: "Is the truth any less true because it is presented in such an unusual manner?" Detailed Descriptions

By the time he revised his testimony about the man photographed in St. Peter's Square, Agca had provided the Italian magistrates with fresh information about his Bulgarian "co-conspirators." He described their apartments, hobbies and facial characteristics as well as listing the telephone numbers through which he said he had gotten in touch with them and the precise dates when they had been in Italy. He also outlined an escape plan involving "a diplomatic vehicle or truck."

When the investigators went to check, most of the personal details provided by Agca about the Bulgarians turned out to be correct. It was later discovered that a truck, specially sealed under international customs agreements, had left the Bulgarian Embassy shortly after the assassination attempt in St. Peter's Square.

The prosecution case is that Agca has succeeded in demonstrating that he knew the three Bulgarian suspects despite their claims to the contrary. And if he can show that he knew them personally, the argument runs, then he can also be trusted when he asserts that they acted as his "controls" in Rome in a Kremlin-inspired plot to assassinate the pope.

Fresh contradictions and inconsistencies in Agca's testimony were, however, in store.

Early in the investigation, according to defense and prosecution sources, Agca told the magistrates about a crucial "planning session" that had allegedly taken place on the evening of May 10, 1981 -- just three days before the assassination attempt. It had been held, Agca said, in the Rome apartment of a Bulgarian airline official, Sergei I. Antonov, arrested by Italian police in November 1982.

Attending the meeting, if Agca was to be believed, were no less than nine people: Agca, the three principal Bulgarian suspects, four other mysterious Turks and Antonov's wife, Rositza. Antonov's 10-year-old daughter, Anna, was also said to be hovering in the background, serving tea.

Defense lawyers representing Antonov seized on Agca's description of this meeting -- which was leaked to the Italian press soon after Antonov's arrest -- to try to prove the innocence of their client. Photocopies of a Yugoslav motel bill and register were produced in an attempt to demonstrate that Rositza Antonov had left Italy two days earlier to drive back to Bulgaria via Yugoslavia. Their daughter Anna was said to have been at school in Sofia all along.

The defense arguments did not impress the Italian judiciary enough to secure Antonov's release -- and the legal value of the documents provided by the Bulgarians to support Rositza's alibi for May 10 is still contested by the prosecution. What is significant, however, is how Agca reacted following the spate of newspaper reports questioning his version of the facts.

On June 28, 1983, he asked to see the magistrates again. He now announced that, contrary to what he had said previously, he had never been inside the Antonovs' apartment. The May 10 planning session had not taken place. He had never met Rositza Antonov and, at the time of the assassination attempt, he had not even been aware that Antonov (whom he said he knew by the code name "Bajramic") was an employe of the Bulgarian state airline, Balkanair.

Court records show that Agca later attributed his "erroneous statements" to a desire "to give greater credibility to my assertions" (about his alleged Bulgarian accomplices).

The prosecutor has accepted as "amazing but in fact probable" Agca's explanation that he had learned the details of Antonov's apartment from newspaper and television reports to which he had access while in prison. A subsequent investigation also showed that Agca had been allowed to consult a telephone directory by a court-appointed Turkish interpreter, Prof. Anna Masala. Telephone numbers that he had provided for his Bulgarian accomplices were all available from this directory. Cross-Examination in Sofia

In July 1983, Judge Ilario Martella flew to Sofia. As the Italian magistrate in charge of the papal investigation, he had already questioned Antonov, who was under arrest in Italy. Now he wanted to cross-examine the other Bulgarian suspects in addition to Bekir Celenk, the Turkish businessman accused by Agca of bankrolling the conspiracy on behalf of the Bulgarian secret service.

Martella was accompanied to Sofia by prosecutor Albano.

The way the Bulgarians tell the story today, the lengthy interrogations by Martella and Albano consisted largely of general questions about their hobbies, interests and actions during the period leading up to the assassination attempt on the pope. Little attempt was made to push them into a corner with Agca's specific accusations about the way the plot was set up.

Italian investigators have since explained that the main purpose of this interrogation was to check the descriptions provided by Agca of his alleged co-conspirators: his recollection that Aivazov had peculiar teeth, that Celenk walked with a limp and had trouble digesting his food, that Maj. Zhelio Vasilev was interested in electronics, and so on.

"They examined me as if I were a horse: height, weight, complaints, diseases, disorders in the teeth," recalled Aivazov, 40, the embassy's former administrative officer. "I told them I had some fillings which they could see for themselves as they looked into my mouth."

Vasilev, 40, the former assistant military attache at the Bulgarian Embassy in Rome, recalled an argument with the magistrates over how his height should be measured -- with his shoes on or off. The exercise was apparently undertaken because of a statement by Agca that Vasilev is taller than Aivazov. In fact, Vasilev is about a head shorter than Aivazov.

Other details provided by Agca about Aivazov included his presence in Sofia in the summer of 1980, his possession of a pocket calculator, a description of his apartment in Rome, his use of a Fiat 124 car, his knowledge of Italian and English, and a visit by relatives to his Rome apartment in May 1981.

Agca's description of Vasilev included the fact that he was in Rome between November 1980 and May 1981, that he was "probably" the military attache of the embassy, that he possessed an electronic watch, that he drove a Fiat 128 car, and that he spoke Russian, Italian and English.

With the exception of vehement denials that they know English -- the language in which they allegedly communicated with Agca -- neither Aivazov nor Vasilev contested these details. Vasilev has, however, argued with another detail that has been cited as evidence that Agca must have known him personally: the fact that the pope's would be-assassin was able to remember a small mole on his left cheek. In his report, prosecutor Albano was later to write that "only someone who had actually seen Vasilev's face from close up" would remember such a detail, which "is not even visible in photographs."

Vasilev attempted to counter this point during a recent interview in Sofia by producing the passport on which, he said, he had traveled to Italy. The passport photograph clearly shows a mole protruding from his cheek. Vasilev added that the passport had been closely inspected by both Martella and Albano.

In a separate interview, Celenk attempted to laugh off Agca's assertion that he suffered from gastric disorders by patting his sizable pot belly.

"I have never had any trouble eating in my life. If I had, perhaps I wouldn't be so fat now," he said.

Celenk also disputed a contention by prosecutor Albano that he possessed a residence permit for Switzerland valid until April 1981 that would have allowed him to enter the country without an entry stamp in his passport. Agca has alleged that, at a meeting in Zurich in March 1981, Celenk promised him and two other Turks 3 million German marks (then about $1.2 million) for killing the Pope. Celenk claimed that his Swiss entry permit expired in 1979 and that he never set foot inside Switzerland during the whole of 1981. Backing Off on Walesa Plot

Back in Italy, at the end of September 1983, Agca made his second significant retraction of evidence. This time it concerned details of a plot to assassinate Lech Walesa, leader of the Polish Solidarity trade union movement, who had visited Rome in January 1981.

In December 1982, Agca had provided precise descriptions of the hotel in which the Solidarity leader had stayed, the hall where he held a press conference, as well as details of his itinerary in Rome. He described a reconnaissance mission to Walesa's hotel in the company of the Bulgarian agents and the preparation of weapons and explosives.

In the revised version of his testimony, as reported by prosecutor Albano, Agca said that the reconnaissance missions had never taken place. He denied ever knowing one of the Bulgarian diplomats allegedly involved in the conspiracy to kill Walesa, Ivan Donchev. The plot against the Solidarity leader, Agca now said, had never been more than a vague project.

Pressed to explain how he knew so much about Walesa's hotel if he had never been there, Agca claimed that he learned the details from magistrates who had interrogated him in connection with a parallel investigation into an alleged Bulgarian spy ring in Italy.

On this occasion, prosecutor Albano rejected Agca's explanations as "unconvincing and in contrast with objective evidence." The magistrates investigating the reports of a spy ring, Albano argued, did not themselves know the details about Walesa's hotel at the time they interrogated Agca.

As a result of his retraction of evidence about the Walesa plot, Agca was formally told that he would be investigated for "libeling" Antonov and the other Bulgarians. The prosecutor has since requested that these charges be dropped on the grounds that it cannot be proved that his original story was false.

Before winding up his investigation into the plot to murder the pope, Judge Martella decided to take Agca back to the scene of the crime: St. Peter's Square. Surrounded by members of the Italian antiterrorist squad, Agca gave the investigators a blow-by-blow account of his actions on the afternoon of May 13, 1981, the day he shot the pontiff.

He pointed out the spot on the broad Via della Conciliazione leading up to the Vatican, near the Canadian Embassy to the Holy See, where Antonov allegedly parked the getaway car, a blue Alfa 2000. He described how he and the Bulgarians had conducted a final scouting of the square where the pope was to appear, paying particular attention to the crowd control barriers. He even showed investigators a shop where he said he had stopped to buy a roll of film.