NBC News scored a clear beat over its rivals at ABC and CBS with its disclosure Saturday on the 6:30 Nightly News that eight members of Congress and other public officials were the objects of a major federal bribery probe. . . .

News executives at both ABC and CBS yesterday conceded that they weren't aware of the story . . . which was unfolding in both Washington and New York . . . until late Friday or early Saturday morning . . . just prior to the time that FBI agents fanned out to notify principals in the probe that they were under investigation . . .

It turns out that investigative reporter Brian Ross of the NBC News bureau here and field producer Ira Silverman out of NBC in New York had known of the extensive "sting" operation for at least two months, piecing the story together from several sources, according to NBC executives . . .

For most of that period . . . Ross and Silverman shared the secret only with NBC News President Bill Small.

But about two weeks ago . . . as it became clear that the federal agents were about to move on the targets of the allegations . . . other NBC News officials were let in on as much as Ross and Silverman knew. . . .

With Small making several trips to Washington for briefings, Sid Davis, NBC bureau chief here, Paul Greenberg, executive producer of the NBC Nightly News . . . and Tom Wolzein . . . head of the Nightly News "special segment" staff were also included in the secret. . . .

Meanwhile, the network learned of the location of the FBI operation in Northwest Washington. NBC rented two large Winnebago vans with large tinted windows . . . special night-vision lenses capable of amplifying available light 30,000 times were dispatched from New York, and round-the-clock surveillance of the home at 4407 W St. NW, used as a base for the sting operation, began . . . A van was parked at each end of the short street located off Foxhall Road . . . and staffed with cameramen . . .

The crews managed to obtain considerable footage of night visitors to the W Street address . . . but it wasn't easy . . . especially after a cold wave hit the area . . .

"We couldn't turn on the heaters because they'd fog up the windows," Davis said yesterday, "and it was so cold in the vans the orange juice froze a couple of nights . . ."

When the word went out late last week that the FBI was about to close in . . . six additional correspondents were brought in from out of town, and a stakeout was set up early Saturday at the homes of all of the congressmen who were due to be notified of their roles in the probe that afternoon. . . .

Thus NBC News was able to have a fairly complete film record of the first major move by federal agents by late Saturday afternoon . . . while ABC and CBS were still scrambling . . . for a story whose dimensions they still weren't certain of . . .

Jane Pauley led the Saturday edition of the Nightly News with about a minute and a half of the story . . . although she still couldn't name the investigation targets . . . and invited viewers to watch a fuller report on the 10 p.m. "Prime Time Saturday" show later that night . . .

The NBC report on Saturday coincided with publication of similar stories in Sunday's early editions of Newsday, The New York Times and The Washington Post . . .

On "Prime Time Saturday," Ross was able to identify by name public figures involved, and the four-minute report included footage of some of the night visits . . . and sketches of the interior of the W Street home used by the FBI . . . Ross and host Tom Snyder then talked another two minutes about legal problems connected with the federal probe . . .

Davis was reluctant to discuss details of Ross' investigations of the so-called "ABSCAM" operation beyond saying that he had relied on "various sources" . . . and that NBC had "known the principals for some time". . . .

The Justice Department announced yesterday that it is starting an intensive investigation of leaks from the agencies involved in the bribe probe. . . .

At CBS News, meanwhile, reporters Bob Pierpoint and Fred Graham had both received hints of a major story involving members of Congress early Saturday morning. . . .

Bureau chief Ed Fouhy . . . attending a CBS meeting on the West Coast . . . was notified and after several transcontinental phone calls the network managed, according to Fouhy "to have all the names on the 6:30 Saturday news" (which is not seen in Washington) . . .

"We had to scramble," Fouhy admitted yesterday "but I think we had some interesting stuff . . . about 3 1/2 minutes on (Sunday) night's program.

"But they (NBC) had some sexy pictures, I'll admit that . . . some of that stuff done at night was pretty good. . . ."

An official of ABC News in Washington said yesterday that "late Friday one of our guys got a tip that a grand jury or something . . some kind of a 'sting' thing . . was in the wind" and after a confirmation early Saturday morning the bureau worked to put together something for the 11 p.m. Saturday news. . . .

"We named all eight men involved in the 11 o'clock news," the executive said yesterday . . . "but more than that, we had some substance in our report . . . what it was all about . . . and I think we mopped everybody up . . ."

Be that as it may . . . reporters from both ABC and CBS News here . . . caught flat-footed . . . were calling NBC News repeatedly Saturday night trying to get the still-undisclosed address for the W Street house that NBC showed on its 6:30 report. . . .

The house next door, incidentally, belongs to former ABC correspondent Margaret Osmer, who now works for the Council on Foreign Relations . . .

ABC News DID manage to get an interview with her for their Sunday night news as she related the strange goings on next door over the past several months. . . .

Ross, by the way, has been an employe of NBC News for 5 1/2 years . . . during which he has probed such major stories as Mafia influence in the movie industry. . . .

Silverman is a 12 1/2 year veteran of NBC. . . .

Both yesterday were reportedly arguing over who would be played by Bob Redford when the movie is made about their scoop . . .