The old guard, the zealots who have been preaching into the wind about cover-ups and conspiracies these many years, had a rare day of vindication yesterday.

Outwardly, they sat in the front row of the circus tent-like hearing room with their overstuffed briefcases, complaining about how the House Assassinations Committee had stolen their research, and fiddled away its millions.

"There's nothing of any substance that has come out of these hearings that wasn't already in the public domain," declared Harold Weisberg, the patriarch of them all, who had boycotted most of the previous months of hearings.

But beneath the crusty exterior of the critics was a quiet sense of inward pride. a feeling of vindication, that slowly and reluctantly their theories and research were getting an official stamp of approval.

"The government is beginning to acknowledge what really happened in Dallas," said Carl Oglesby, co-director of the Assassination Information Bureau, a nonprofit clearinghouse.

The star of the day's show, Mark Weiss, a professor with a salt and pepper beard and ramrod straight back, was not one of their own. He is a scientist, one of the acoustics experts whom Judge John Sirica called in to listen to the Watergate tapes.

In the world of lawyers and congressional hearings, scientists with big bucks to spend always have more credibility than freelance investigators who operate on a shoestring, regardless of what they have to say.

The nuts and bolts of Weiss' startling findings -- that new tests had found that a second gunman had fired a shot at President Kennedy from a grassy knoll in Dealey Plaza in Dallas on Nov. 22, 1963 -- leaked out a week ago. But now he was performing for the record at a specially called hearing after the committee had all but completed its work.

Weiss performed well, very professionally, not giving an inch as he said there was a "95 percent probability" that two gunmen had shot at Kennedy in a crossfire. If he were a lawyer, he quickly added, he'd say the evidence was "beyond a reasonable doubt."

This, of course, is what many conspiracists have been saying for years. Weisberg first wrote about it in his 1965 book "Whitewash I." David Lifton, another well-known critic, reached a similar conclusion in a 1967 article in Ramparts magazine, although he maintained there were three, not two assassins, involved.

Lifton found a certain irony in it. "None of this is new to us [conspiracy theorists]," he said. "But look at it this way. It's dignified for a lawyer to stand up in front of a room and call on an acoustics expert. It's like an event that's been playing for years has finally gotten a sound track."

It all added credibility to Lifton and the rest. He was beaming. A publisher, he said, had finally agreed to print his book about medical evidence in the case, which he's been working on for 10 years. Oglesby was also in demand, providing color commentary on the Public Broadcastiing System.

"It's hard to find a word to describe our feelings," said Oglesby. "You don't, want to say, 'I've been vindicated.' Nobody can delight in proving the fact that the country was badly misused by its institutions. But then, it's good to see it moving in he right direction again."

But there was a certain beleaguered resentment among the researchers and theorists, many of whom have devoted years of their lives to investigating the assassination. Many of them hold little truck with their fellow critics, sven less with the House Assassinations Committee.

"It's a miracle that so much evidence in the case has been turned up by a group of freelancers working on a shoestring," Lifton said. "We couldn't be in this room if it hadn't been for the freelance investigators."

The assassination community, as the researchers and theorists have come to be called, was an odd lot as it gathered yesterday.

Each member carried a briefcase, stuffed with newsletters and documents to use as "proof" that his work was the first and the best on the subject.

Weisberg, who has spent most of the last 15 years investigating the Kennedy assassination, sat in the front row, his feet on his briefcase, a wry smile of contempt on his face as he slipped notes to reporters on the fine points of the case.

On one side was his lawyer, James Lesar, the attorney for John Ray, brother of James Earl Ray, convicted of killing the Rev. Martin Luther King Jr. On the other was Dr. James Barger, the committee's medical examiner, whom Weisberg has called "a liar."

When the committee broke for lunch, the conspiracists gathered around the witnesses, pitching questions at them, and complaining among themselves. "I made those pictures they were showing up there," complained Robert Groden, a 32-year-old photographic critic. "And here they're not giving me any credit."

Weisberg, the dean of them all, held back, talking only to a few allies and reporters. A former champion chicken farmer and Senate investigator, he has worked for years on the assassination from his lonely, Waldenesque home in Frederick, Md., alone and with little monetary gain or recognition.

He does his won investigating, prints his own books (there are six of them ranging from "Whitewash I" to "Post Mortem"), and distributes them all from his home, sending a couple out every day by mail.

If there's a bitterness in his voice, it's understandable. He has lived on a shoestring, financing his own investigations. The House Assassinations Committee ignored him, he said yesterday.

The only time anyone from the committee got ahold of him was to ask for a couple of his books, he said. And then he had a devil of a time getting the panel to pay for them.

"The only things they've come up with that weren't on the public record are fabrication and conjecture. It's a tragedy," he said. "This case has been one of the greatest causes of disenchantment and distrust wiht our government. And this committee is just making it worse."