Leonid Brezhnev looks up from his briefing book to signal that he is through.

He has heard President Carter unveil a sweeping vision of strategic arms limitations treaties (SALT) in the 1980s - a futuristic sweep that ranges from steep reductions in missiles to "safe havens" for submarines. Brezhnev has responded, in turn, with a more traditional reading of the Soviet line on SALT III. He is a visibly tired and failing man; they are talking about a subject he probably will not live to see through.

"...and so," Brezhnev is saying, "I'd like to conclude my talks on the arms race...."

Carter is looking directly across the table at this Soviet leader who signed his first SALT agreement back when Carter was signing orders for filling potholes in the state roads of Georgia.

"We will consider your statements," Carter begins, "but first I want to go back to the issue of the Backfire bomber." This is an issue the Soviets thought they had resolved the previous day.

And with that, Carter embarks on what is to be the only moment of real confrontation at the Vienna summit.

It is Sunday morning, the second day of talks. That Sunday morning meeting proves to be a good time to measure the work of Carter at the summit. It is a morning in which he:

Presents the Soviets with an ambitious view of how the coming generation of nuclear super-hardware and technology can be curbed in the next decade - a vision of SALT III that intrigued the nuclear engineer in him perhaps even more than it did the summiteer.

Prods the Soviets into finally holding a side meeting at the summit between the U.S. and Soviet defense ministers and military chiefs - the first such meeting since the Cold War; and a session that is especially important because these are the men who will likely be running Soviet military policy long after Brezhnev is gone.

Presses until he finally gets his way on a formal understanding concerning the controversial Soviet Backfire bomber - but only after a slip-up the day before that had enabled the Soviets carefully to hedge their statement of assurances.

The proceedings of that crucial Sunday meeting have been reconstructed from the notes of officials who were sitting on the American side of the table. Any accounts or conclusions of these events must be viewed with historical caution. After all, a reading of the columnists and news analysis that followed the 1961 meeting in Vienna between John F. Kennedy and Nikita S. Khrushchev showed that not even those who had the benefit of a private post-summit interview with the president adequately picked up on what would subsequently become clear - that Khrushchev had come away judging Kennedy to be so irresolute that he would later challenge the American president by placing Soviet missiles in Cuba.

Instant history has its faults, but it also serves a purpose. And from what can be known now of the 1979 Vienna summit, it appears that Carter achieved what he set out to do in his baptism of summitry.

But just a few hours before he began that eventful Sunday morning session, Carter realized that the outlook of the summit had suddenly turned unclear.

There had been a slip-up at the summit table Saturday. When Carter had been driven back to the U.S. ambassador's residence after his first day of talks with Brezhnev, he was informed by Defense Secretary Harold Brown and national security affairs adviser Zbigniew Brzezinski that the exchange of assurances on the Backfire bomber had not worked out as planned.

The plan had called for Carter to make introductory remarks on the issue of the Backfire - a bomber that the Soviets maintained would not be based for use against the United States.

According to the plan, Brezhnev would then hand Carter a written statement that would include the assurance that the Soviets would not increase production of the backfire above its "present rate."

Carter was then to hand the Soviets a written statement saying it was the U.S. understanding that the present rate of Soviet production of the Bakcfire was 30 per year. And Brezhnev was then to agree orally, for the record, that the production rate was 30.

So finely tuned was this overture that just prior to the summit Secretary of State Cyrus R. Vance and Soviet Foreign Minister Andrei Gromyko had negotiated over one word. The Soviets had wanted the American document to say the production rate was "approximately" 30 but Vance insisted upon dropping the word "approximately" and Gromyko finally agreed.

Such was the script. But this is how the act really played on Saturday:

It was Brezhnev who made the opening comments on the issue of the Backfire and handed Carter the written Soviet statement, which was worded precisely as had been agreed. But Carter decided at that moment not to hand Brezhnev the written U.S. statement. (He did this, U.S. officials say now, because part of what was in the written U.S. statement had already been said by Brezhnev.) Instead, Carter presented the rest of the U.S. Backfire position orally - including the production figure of 30.

Brezhnev said he had "no rebuttal" to the U.S. statement. Carter replied: "I understand that you have agreed that the production would not exceed 30."

Only later did American officials realize that Carter had left the table Saturday without actually getting Brezhnev's assurance limiting production to 30. "We looked over our notes on it," recalled one official who was at the table, "and it was clear that we had a problem."

On Sunday morning, Carter met with his top advisers at the U.S. ambassador's residence and drafted a new script on how he inteneded pinning down the Soviets on a specific commitment on the Backfire production figure.

The Backfire had been a controversial issue - mostly among hardliners in American - ever since the then-Secretary of State Henry A. Kissinger had agreed at Vladivostok in 1974 that the Backfire would not be part of the SALT II agreement because it is of limited range.

Carter went to Vienna with a harsh sendoff from Sen. Henry M. Jackson (D-Wash.), whose warnings of "appeasement" and "surrender" reverberated from Washington to Vienna. Jackson's statements clearly stung the Carter officials.

So on this Sunday morning, Carter decides he will not back down on assurances for the Backfire - not on one word or one number.

Carter is far from certain what the outcome of the morning will be as he double-times up the steps of the Soviet Embassy for the Sunday talks.

It is 11:04 a.m. when he begins with what his aides have hoped would be viewed eventually as the showpiece of his performance at the Vienna conference table - his view of SALT III. For the moment, he is holding his fire on the issue of the Soviet bomber.

Carter opens by saying that SALT II is a significant accomplishment, but does not go far enough. It permits a large buildup of warheads. The United States hopes to pursue an agreement that will end the arms race, he says. There will be a great waste of material resources by both sides if each continues to match the other - and yet that is what seems to lie ahead.

"You are concerned with our cruise missiles," Carter says. "We are concerned with your large missiles...[and buildups in] civil defense...air defense."

Carter talks of the need to improve verification through pre- otification, improving monitoring a ban on encryption of telemetry data, and perhaps on-site inspection.

The United States is prepared for an agreement in large reductions in missile "throw-weight," in warheads, and a moratorium on the deployment of any new launchers and missiles, he says.

As a prelude to SALT III, he says, the United States is prepared to negotiate reductions below the new SALT II levels.

As the numbers of weapons are reduced, Carter says, both sides must be prepared to make existing weapons less vulnerable.

Here, Soviet Defense Minister Dmitri Ustinov interjects to ask if this doesn't run counter to Carter's earlier statements about the need to restrict technological escalations of the arms race.

No, Carter replies, the point is that each side should place its efforts on ensuring the survivability of its existing forces, rather than on destroying the other side's forces.

This is the line of reasoning that supports the decision to switch to mobile missiles, which would be less vulnerable to being wiped out by an enemy first strike.

But Carter goes farther than that. He says that one possibility would be to create a "safe haven" for submarines armed with nuclear warheads. This would create an area of the ocean that would be excluded by mutual agreement from antisubmarine warfare.

Although Carter's dissertation is far-ranging, he makes no mention of U.S. and allied nuclear weaponry based in Europe - the so-called forward base systems. The Soviets insist this must be part of any SALT III discussion (it originally came up as a Soviet demand early in SALT talks, but then was dropped).

As expected, Brezhnev, in his discussion of SALT III, puts emphasis on force reductions in Eruope. Carter interjects to say that he is interested in what Brezhnev is talking about and suggests that the defense and military chiefs hold a side meeting to talk about it. This is a bit of deliberate U.S. scripting to get the military talks going.

The Soviets confer. Brezhnev is not anxious. He suggests that there are procedural matters to resolve first.

"I don't have a preference," Carter persists. "Why don't we have the military leaders meet?"

There is a second Soviet conference and finally Brezhnev announces that he has "no objections." Soviet military chief of staff Nikolai Ogarkov and U.S. Chairman of the Joint Chiefs of Staff Gen. David Jones exchange quick notes. As Brezhnev brings his own SALT III statements to a close, the military leaders agree to meet during lunch.

Now Carter makes his move to reopen the Backfire issue. On the American side - where everyone knows what is coming - there is a feeling of tension as Carter begins.

"We had an agreement when we came here that you would confirm our production figures on the Backfire," he reminds the Soviet negotiators.

Gromyko says that there is "no problem," and recalls that he and Vance had agreed earlier to omit "one word" (the word "approximately").

Now Vance interrupts: "Yes, but you also agreed you would confirm the figure of 30 per year." The secretary of state then turns toward Brezhnev and asks: "Do you confirm that?"

Brezhnev does not confirm it. Instead, the Soviets huddle and exchange comments.

Carter does not wait before pressing his case. "Mr. President," he says to Brezhnev, #we had an understanding before you came here that you would confirm those figures. We had an understanding - and it was on this basis that we would sign the treaty."

The room falls silent. Carter's implication weighs heavily across the conference table.

Vance again asks: "Do you agree that the production figure is 30?"

Carter switches to a different formulation of the question. He asks: "Do you agree you will not produce any more than 30 a year?"

And Brezhnev sits back, raises his hands from the table, and yields. "There is no problem," the Soviet leader says. "There is between us an understanding that we will not produce more than 30 a year."

EPILOGUE: Just how far Carter intended to press for assurance on the Backfire production limitation is unclear. "It was a serious matter - a very serious matter," says one of the senior officials on the American side of the table.

Would Carter have actually refused to sign the SALT II treaty over this issue?

"I don't want to say," says this official. And two others from the American delegation say they just do not know.

On the face of the affair, it seems hard to believe that Carter would not have signed regardless of how the Backfire discussion ended. But one official who sat at the table is not so sure. He notes that Carter had gone to Vienna feeling he had in the past not been told the truth by Soviet officials (concerning Soviet generals and Cuban troops in Africa) and this was a case where he felt the Soviets were going back on a specific pre-summit agreement.

"I think that the president felt that this matter had a great deal to do with his very relationship with the Soviet leadership," this official says. "And in that sense, it was more important that the Backfire or even SALT II itself."