The Security Council again postponed a vote on economic sanctions against Iran tonight, although U.S. Abassador Donald McHenry said he was "99 and 44/100 percent sure" that a new Iranian initiative on the hostage crisis offered "no substantive change in the situation."

McHenry said the Iranian government has sent "a letter and not a response" to Secretary General Kurt Waldheim's request for clarification of a compromise proposal verbally presented by the Iranians yesterday. The letter arrived barely 30 minutes before a scheduled 6 p.m. sanctions vote. t

At 8:30, the council began a closeddoor session, only to emerge 2 1/2 hours later with the anouncement that it had taken no action, but would hold another meeting at 6 p.m. Sunday. Neither McHenry nor other council delegates would say if that meeting would be a sanctions vote or merely further consultations.

McHenry insisted that the further postponement represented no backdown from previous U.S. insistence that the American hostages held in Tehran since Nov. 4 must be released or the United States will press for sanctions vote. At the same time, however, he said it would be wrong to interpret the two successive postponements as a sign of substantial progress.

"To leave high expectations . . . would in my judgment be a cruel hoax on those families and relatives [of the hostages] who may look to these actions as more significant than they are," McHenry said.

But "it would by irresponsible of us not to pursue the slightest possibility of resolving this problem," McHenry said. "We hope to continue in a responsible matter and follow any avenue which may possible lead to a settlement."

Still, McHenry said unequivocably, "there in no movement."

Other Security Council members differed with McHenry's interpretation and said the Iranian proposal did offer a glimmer of hope. Many of them have expressed reluctance to vote in favor of sanctions against a fellow Third World country and are more receptive to any possibility that will delay what they feel is a painful choice.

Throughout the hectic, and often secretive diplomatic endeavors here over the past two days, the following picture emerges:

Shortly before a council meeting scheduled to vote on sanctions yesterday, Iran's delegate to the United Nations, Mansour Farhang, visited Waldheim and told him he had authority from Foreigh Minister Sadegh Ghotbzadeh to discuss Waldheim's offer -- made during the secretary general's Jan. 2-4 visit to Tehran -- for a U.N. commission to investigate Iranian charges against the deposed shah and U.S. involvement in Iran.

Iran, Farhang said, wanted U.N. recognition of the legitimacy of its claims on the shah and his fortune. This was the first indication that Iran might be prepared to accept recognition of the rightfulness of its demands rather than the actualization of them.

Farhang made no guarantees concerning the release of the hostages.

According to an informed Asian diplomat, Waldheim asked if Ghotbzadeh had reveived the approval of Iranian revolutionary leader Ayatollah Ruhollah Khomeini to make the offer. Following consultations with Tehran, Waldheim was told that Ghotbzadeh had visited Khomeini in the Shiite Moslem holy city of Qom to discuss the proposal.

Waldheim transmitted the proposal to McHenry, who agreed to a postponement of the council session but said that the sanctions vote must be held by 6 p.m. today if the offer had not been sufficiently clarified. The United States wanted to know by what authority the offer was made and specifics concerning the release of the hostages.

The Iranians reportedly said they would send no direct written communication either to the Security Council or the the secretary general. Waldheim then drafted a written outline of his interpretation of the Iranian offer and included the U.S. demands for clarification. At 1 a.m. today, he cabled the message to Ghotbzadeh, reminding him that the council meeting was scheduled for 6 p.m.

Barely a half hour before the meeting was to begin, Waldheim received what McHenry and others described as "a letter," presumably from Ghotbzadeh, and transmitted it to McHenry.

The letter reportedly repeated acceptance of Waldheim's proposed investigation and then said Iran wanted results of that investigation to be submitted to the United Nations. It again made no direct reference to a specific time frame for release of the hostages.

According to diplomats, included among the wide range of things remaining to be clarified are whether the hostages could be released when a U.N. commission is appointed, when such a commission would arrive in Tehran to begin an investigation, for when the United Nations would act on its findings. At the same time, the diplomats said the Iranians displayed "limited knowledge" of U.N. procedures and did not specify whether a commission report should be presented to the Security Council, the General Assembly, or to the secretary general, and what should be done with it by any of those authorities.

In the most positive interpretation of the Iranian letter, Arab League representative Clovis Maksoud said it had "reformulated" Waldheim's earlymorning cable with some "positive ambiguities."

Waldheim, in a brief statement before he entered tonight's council meeting, said that "the United States wishes further clarification" of the proposal and noted that he, too, believed it "merits further clarification."

McHenry's interpretation was the most pessimistic. The letter, he said, contained "curious wording" that, in addition to indicating "no movement" on specifics, contained no higher authorization than yesterday's original proposal. This presumable meant he did not feel the letter had been cleared with Khomeini.

McHenry insisted that the United States is still assured of the nine council votes necessary to pass the sanctions resolution. The nine-vote majority, however, would bring only a "moral" victory for the resolution, since the Soviet Union is believed certain to veto it.

Several weeks ago, when sanctions were first plroposed, the United States hoped for a Soviet abstention. Following the Dec. 27 Soviet-backed coup in Afghanistan, however, and exchange of harsh rhetoric between Washington and Moscow, the Soviets are now considered to feel they have nothing to lose vetoing the Iran resolution, and something to gain in terms of potential friendship with Iran.