Investigators trying to solve the mob-style slaying of Nat Masselli are focusing on a social club in the south Bronx where some two dozen men supped on linguine the night he was killed.

Among those present, sources say, were two reputed mobsters who had been involved in the investigation of Labor Secretary Raymond J. Donovan along with Nat Masselli and his father.

Presiding at the get-together, investigators say, was Philip Buono, a "top guy" in the Genovese family of the Mafia.

Buono has also been identified in FBI reports as a key backer of Nat's father, William Masselli, when Masselli became a multimillion dollar-a-year subcontractor on a series of New York City subway projects for Donovan's firm, the Schiavone Construction Co.

Sources said others present at the meal included Joseph (Hooks) Verlezza, 71, a close associate of Buono, and Salvatore (Sally O) Odierno, 66, a reputed Mafia "soldier" who has since been arrested and indicted for Nat Masselli's murder that night on a well-lit street near Van Cortlandt Park.

The meeting at the social club at 151st Street and Morris Avenue in the Bronx reportedly ended around 6 p.m. on Aug. 25.

Masselli, meanwhile, was visiting his father at the Metropolitan Correctional Center where the elder Masselli, now a 56-year-old convict, was waiting to be called before a federal grand jury in the Donovan case.

Nat Masselli left his father at the downtown Manhattan prison around 6:15 p.m.

According to one law enforcment source, he then stopped outside the social club and spoke to at least "a couple of people" who were still there. According to several sources, Masselli chatted with Buono and Verlezza.

The 31-year-old Masselli then drove off in his Lincoln Continental and wound up parking it in a residential neighborhood on Van Cortlandt Park East, just in front of Odierno's red Pontiac.

Two men entered his car, one sitting beside Masselli in the front and one in the back, according to investigators. He was shot behind the right ear at 8:21 p.m., apparently just as he was attempting to drive off.

Witnesses say his two assailants, one identified as Odierno, then fled in the red Pontiac which was driven by a third man who had remained at the wheel.

Odierno has been identified in a police lineup as one of the men who scrambled out Masselli's two-door Lincoln, but no other arrests have been made.

Odierno's car was found in Brooklyn a few days later but, sources say, it did not even have Odierno's fingerprints in it, much less those of any other suspect, by the time investigators got to it.

"There's a hell of a lot of evidence out there, but it's very difficult to get people to talk," says Bronx District Attorney Mario Merola, who has been directing the investigation with the help of FBI agents concerned about possible obstruction of the Donovan investigation.

Sources say investigators seeking to determine who was with Odierno have Buono and Verlezza "on the list" although they are not the only ones on it.

In any case, Buono and Verlezza were among the organized crime figures who had been summoned by special prosecutor Leon Silverman last spring to testify about alleged ties to Donovan.

According to his report in June, Silverman wanted to question Buono and Verlezza about still-unresolved assertions that Donovan met in Miami in January, l979, with William Masselli and perhaps other members of the Genovese crime family to discuss construction jobs in the New York area.

Verlezza said he was too ill and never testified. Buono apparently said nothing significant.

Silverman reopened his probe in July when additional allegations came to his attention, among them, sources say, new reports about events in Miami. The prosecutor decided to question Buono and William Masselli again and to press for testimony from Verlezza, among others.

In fact, Masselli, who began serving a seven-year term on federal hijacking and narcotics charges last winter, had been brought to Manhattan from an upstate prison just one day before his son was killed, in response to a writ Silverman had obtained.

Despite such circumstances, however, Silverman has decided that there is no connection between the murder and his renewed inquiry.

"The causes," he insisted last week as the homicide investigation was just warming up, "will be found to be otherwise."

Silverman said he did not think Masselli's death "was associated with the Donovan investigation at all."

In the past, the special prosecutor had avoided such pronouncements before the evidence was in. He said he felt he had to speak up in this instance "because people were going around saying I was responsible for Nat Masselli's death."

Such suggestions have been made because Silverman disclosed in his June report that William Masselli tried to cooperate with federal investigators last May and establish that he had paid a $20,000 kickback to an official of Donovan's construction firm in return for a $20,000 loan. His son, Nat, had helped FBI agents record his phone conversations with a Schiavone lawyer about the subject.

"He Silverman doesn't want a connection because that would mean he'd have to stay in business," suggested one source familiar with the inquiry.

District Attorney Merola has indicated that a dispute over the proceeds from William Masselli's subcontracting business may have triggered Nat Masselli's death, but he also said it could have been caused by a combination of factors. Investigators believe something unexpected that Nat Masselli said as he sat in the car must have touched off the shooting.