U.S. negotiators of a new strategic arms limitation treaty (SALT) completed preparations yesterday for an important round of Moscow talks displaying notable and surprising, caution about the chance for early success.

The cautionary notes, including some estimates of a 50-50 chance of reaching final agreements in principle in Moscow, and other estimates of much [WORD ILLEGIBLE] chances, contrasted [WORDS ILLEGIBLE] which prevailed [WORDS ILLEGIBLE] years ago.

[WORDS ILLEGIBLE]I reserve, displayed as [WORD ILLEGIBLE] team headed by Secretary of State Cyrus Vance met for a final review before flying to the Soviet Union today, also contrasted with now confirmed reports of recent large-scale compromises on most of the issues which previously blocked a SALT II pact.

There was no way for reporters to be certain whether the caution reflected uncertainty on the eve of a decisive moment, new difficulties, a negotiating gambit or simply a desire to hedge against unfulfilled public expectations.

The Moscow negotiations follow by only three weeks successful talks in New York and Washington with Soviet Foreign Minister Andrei Gromyko and his team.

The unusually brief interval between the two rounds of high-level meetings was widely seen as an attempt to capitalize on forward motion.

The remaining issues to be tackled this weekend are described as diffcult because of their political significance rather than because of their strategic importance or technical difficulty. Compared to the problems which recently have been solved or are on the way to solutions, the final set of problems seems relatively minor.

Since Vance and Gromyko and their negotiating teams sat down here in Geneva in early July the following progress has been made on major issues:

The superpowers have agreed to limit their land-based intercontinental nuclear missiles to one "new type" of improved weapon on each side, and to extend this limitation to 1985 (instead of only three years as previously discussed).

They have also agreed that "new types" of submarine-launched nuclear missiles will not be limited during the treaty period.

The U.S. and Soviet negotiators agreed to limit the number of independently targetable multimple nuclear warheads (or MIRVs) that can be fired from any one missile. Without these MIRV restrictions, which vary in number according to the test record of various missiles, the mushrooming growth of multiple warheads on big rockets could make restrictions on the total missile numbers almost meaningless.

The Soviet Union, in a concession to the United States, has dropped its demand for range limitations on air-launched cruise missiles. Previously the Russians held insisted, and the Americans tentatively agreed, that testing and deployment of these highly accurate airborne weapons would be limited to 2,500 kilometers (about 1,600 miles) in range. Difficult negotiations were taking place about the methods of calculating the limit, in an effort by the American side to stretch the range to ensure that American weapons could reach all important Soviet targets.

The United States, in a concession to the Soviet Union, has dropped most of its demands for detailed assurances on the production, basing and refueling of the Russian Backfire bomber. In a tactic intended for the Joint Chiefs of Staff and U.S. Air Force as much as for the Russians, the American side is prepared to state that it reserves the right to deploy a future manned bomber similar to Backfire. Neither bomber will count against the strategic arms limits of SALT II.

Some of the concessions and accommodations, according to American officials, have been offered on condition that agreement is reached on issues that are still outstanding. Thus some of the tentative deals have "a string" tied to them and cannot yet be counted as fully agreed.

The list of problems remaining to be resolved in the Moscow negotiations, in view of the 50 pages of previously agreed treaty text, is hort and limited in scope. The outstanding issues, as described by informed sources, include:

The range of ground and sea launched cruise missiles. In return for dropping range limitations on the more promising air-launched cruise missiles, the Soviets are proposing strict enforcement of a tentatively agreed provision to limit the deployed range of the other versions of the cruise to 600 kilometers (about 400 miles). And the Russians want to extend the time limit on this range restriction to 1985 rather than only period of the three year "protocol."

There are some indications that this is the most difficult remaining issue, in view of the interest on the part of several NATO allies in these cruise missiles of the future. European leaders are reported to have expressed their views on this point to presidential assistant Zbigniew Brzezinski during his trip to Germany, France and Britain following the last round of SALT talks with Gromyko in Washington.

The numbers of air-launched cruise missiles to be permitted on any one aircraft. B52 bombers can carry a maximum of 20 such weapons, but some Pentagon strategists have studied plans for bigger jets to carry a larger number. There are indications that a compromise on this issue is in the making.

The expiration of the treaty "protocol," originally planned for three years, covering several of the most sensitive issues. And a related matter, the date by which the Soviets must reduce their overall nuclear arsenal from 2,400 to a new limit of 2,250 strategic nuclear launchers.

A few relatively exotic issues, perhaps including a possible exemption from SALT or cruise missiles which are amred with conventional rather than atomic warheads, if a way can be found to tell them apart with assurance from afar.

According to some sources, the exact terms of a "statement of principles" intended to guide the superpowers in negotiations toward greater reducations and restrictions in a future SALT III pact.

Officials here dispute reports that the U.S. insistence on the right to build a future "shell game" mobile missile system is a serious stumbling block. The United States has asserted that nothing in the treaty as drafted forbids such a system if the number lf launchers involved can verified by spy satellites.

According to critics of the treaty, the Soviets have expressed the view that such a hypothetical system would be "deceptive" and in this case, ruled out by the agreement.

There is little doubt that, with the will to succeed and a spirit of accommodation, United States and Soviet leaders could solve the remaining problems during the forthcoming Moscow meetings and thus set the stage for a summit conference of President Carter and Soviet leader Leonid Brezhney to sign the accords later this year.

The guarded comments of Americans negotiators preparing for the new round of talks raided doubt that, after nine years of negotiation, agreement on a full-scale treaty limiting offensive nuclear armaments is at hand. The officials said there is no way to be certain of the outcome, however, until the discussions with Soviet leaders unfold the Kremlin over the next several days.