Early in the afternoon of last Nov. 23, a Sunday, White House aide Oliver L. North arrived at the office of Attorney General Edwin Meese III to answer official questions for the first and -- to date -- last time about the diversion of profits from the secret U.S. arms sale to Iran to aid the contras.

The questioning centered on the closest thing to a "smoking gun" that has yet emerged from six months of investigations of the Iran-contra affair: the "undated memorandum," a National Security Council decision paper for the president, drafted by Lt. Col. North in April 1986 at the direction of the national security adviser at that time, then-Vice Adm. John M. Poindexter.

A version of the memo, which included a proposal to divert $12 million from one of the arms sales to Iran, had been discovered the previous day by Justice Department officials during an internal review of the Iran initiative.

President Reagan has denied all knowledge of the diversion, and a linkage between him and the "undated memorandum" has not been publicly established. Such a connection now would be a devastating revelation that he had deliberately circumvented the congressional ban on U.S. military aid on a large scale.

According to a little-noticed summary of the Meese-North conversations released last week by Sen. Warren B. Rudman (R-N.H.), vice chairman of the Senate select committee investigating the affair, North behaved that day in a way that strongly suggested he was trying to protect the president, and may have believed that the incriminating memo had been destroyed.

Two days earlier, at his office in the Old Executive Office Building, North had destroyed a foot-and-a-half stack of highly classified documents and altered others that implicated North and the White House in secret support of the Nicaraguan contras, according to the testimony of his former secretary, Fawn Hall.

When Meese informed North that there was "evidence" of a diversion of funds, North "registered surprise," according to the summary, which was based on what Assistant Attorney General William Bradford Reynolds told House and Senate investigative committees last February and April.

On learning that the Justice Department officials had "evidence" of a diversion, North asked if there was "documentation," and was then shown the memo.

At that point, North asked if there was a "cover sheet" -- the transmittal letter that usually routes top secret "action" memos to the national security adviser and the president. Strangely, the summary does not record what the Justice Department officials answered about the "cover sheet." But investigators for the Tower review board and the congressional committees say they have never found such a piece of paper.

"North seemed quite surprised that the produced files contained a document with a reference to the contra diversion," according to the summary. "Reynolds had the impression that North had reviewed the NSC documents before they were made available to the DOJ {Department of Justice} reviewers on Saturday."

Only after those exchanges did North volunteer that he had drafted the memo and tell the Justice Department officials that "the memorandum had been shown to Poindexter, but that it was not to be circulated beyond Poindexter."

By then, however, if Reynolds' account is complete, North would have known that the Justice Department officials did not have a final copy of the memo. He would have known that they did not have any potentially incriminating cover sheet. And he would have established that the document was only a draft: It contained a handwritten notation, "April," and the date Sept. 14 had been lined through and "13" scrawled over it.

Since then, the congressional investigation has only raised more questions for the president about the diversion memo.

Last week, during testimony by Fawn Hall, it was disclosed that a later copy of the memo exists that carries the corrected Sept. 13 date. That version was found on one of Hall's computer floppy disks, but no printed copy has been located, according to investigators.

Hall confirmed that the memo had gone to Poindexter, been revised, sent back and redone. She also said she assumed that the memo was "typed for a reason and that it went somewhere." But the cover sheet that North inquired about has not turned up in either the NSC files or computer records -- at least according to public knowledge.

It is not known what information investigators have elicited since they began questioning Poindexter in secret sessions early last month. According to sources close to the Tower review board, Poindexter's files at the NSC contained neither the key Iran-contra documents nor any of the notes that his associates said he took copiously while at the NSC.

But the report of the Tower review board and the testimony of several major witnesses before the House and Senate investigating committees have created an intriguing record of North's concern with the April 1986 memo.

Only hours before going to Meese's office on Nov. 23, North stopped by the K Street office of his friend and previous boss at the NSC, former national security adviser Robert C. McFarlane, for a discussion that touched on the memo.

North "thought that the president was in a very solid position," McFarlane told the congressional hearings. But McFarlane said there was something that "concerned" him: "It was the matter of the channeling of funds to the contras from the Iran account."

McFarlane quoted North as saying that the diversion had been "approved" and that it was "a matter of record in a memorandum that he had done for Poindexter." McFarlane then recalled North saying "something like, 'I must do -- see what can be done about that memo' -- or words to that effect."

On Nov. 25, North was fired and Poindexter allowed to resign after he, too, had admitted to Meese that he knew of the diversion scheme.

On that same day, McFarlane has testified, he telephoned North while on a trip to London to offer his consolation:

"I asked again, 'Ollie, it was approved, wasn't it?' And he said, 'Yes, Bud, it was approved. You know I wouldn't do anything that wasn't approved.' "

McFarlane recalled asking North what happened and being told, "They must have found the memo."

"Did he say, 'I missed one?' " Senate chief counsel Arthur L. Liman asked.

"Something like that," McFarlane testified.

There is a note of irony in the fact that the document turned up during a review ordered by Reagan; in the months since then, the credibility of Reagan's presidency has rested on his repeated assertions that he knew nothing about the diversion.

Since last November, when Reagan said that there was "no foundation" to a published report of U.S. arms sales to Iran, the only major assertion from which he has never been forced to retreat is his assertion of ignorance about the diversion.

On the morning of Friday, Nov. 21, Poindexter sat in on a meeting at which Reagan directed Meese to pull together the information on the Iran arms sales. Meese told Poindexter he would send Reynolds and other officials to the NSC the next day to review North's files on the Iran initiative.

Toward noon, North arrived at the suburban Maryland house of NSC consultant Michael A. Ledeen and found McFarlane already there, according to McFarlane's testimony. But North departed quickly, after saying "something has come up" that required him to return immediately to the office. McFarlane offered him a ride, and during the trip North confided to his old boss that there was going to be a "shredding party."

At 1:30 p.m., according to North's appointment book, North met with Poindexter and then returned to his office, Suite 302 at the Old Executive Office Building. According to Hall, he directed the alteration of some documents. Meanwhile, he also opened the five-drawer safe that contained his personal papers and selected documents to be shredded. Whether the final copy of the diversion memo may have been among those destroyed is a question that has not been answered.

The next morning, the undated version was discovered inside the files provided to Reynolds by the NSC. Also discovered, according to Justice Department sources, were other versions of the memo that did not contain references to any diversion. A source close to the Tower review board who has seen them described these versions as "altered."

Those documents have not, however, been introduced into the record of the congressional hearings.

Reynolds advised Meese about these discoveries at lunch, and that afternoon Meese arranged for North to go to his office the next day.

That night Meese stopped by the house of then-Central Intelligence Agency Director William J. Casey, but says he did not mention the discovery of the diversion memo.

Despite Reagan's repeated statements that he was unaware of any diversion as proposed in the memo, the information in it was intended for him. According to an April 7, 1986, computer note from North to McFarlane, the document was prepared by North at Poindexter's direction "for our boss," a phrase that Fawn Hall last week described as meaning the president.

The document described past arms-for-hostage deals with Iran and outlined steps that could lead to a high-level U.S. mission to Tehran, in which the sale of additional U.S. arms to Iran would be contingent on the release of all American hostages held in Lebanon by pro-Iranian extremists. Attached to the version of the memo obtained by the congressional committees were "terms of reference," dated April 4, 1986, the guidelines to be used by the official American delegation in the proposed "U.S.-Iran dialogue."

McFarlane has testified that the "terms of reference" were approved by the president prior to the May 1986 mission to Tehran that he headed. Other copies of the "terms of reference" have been found in NSC files, suggesting Reagan could have received them without getting the "undated memorandum."

Several of the proposals in the undated memorandum -- including the diversion of the funds itself -- were implemented, thus giving credence to the belief that the president authorized the plan.

The memo recommended, for example, "that the president approve the structure depicted" for taking steps to exchange arms for hostages.

On April 8, Iranian middleman Manucher Ghorbanifar sent a message to CIA official Charles Allen that he was working with his Iranian contacts to carry out arrangements mentioned in the diversion memo.

On April 16, North and Poindexter exchanged computer messages about a preparatory session between North and the Iranians in Frankfurt that required presidential authorization. The session eventually was postponed, but had been set to go ahead, indicating it had received presidential approval.

Poindexter wrote North: "You may tell them {the Iranians} that the president is getting very annoyed at their continual stalling. He will not agree to any more changes in the plan."