The key undercover agent in the FBI's Abscam operation yesterday in federal court here that he was mistaken when he testified last week that no recording equipment was available to tape two telephone conversations in which a meeting was arranged between Rep. John W. Jenrette Jr. (D-S.C.) and representatives of a phony Arab Sheik.

The undercover agent, FBI man Anthony Amoroso, was unexpectedly recalled to the witness stand yesterday at Jenrette's trial in U.S. District Court on conspiracy and bribery charges. Amoroso was vigorously cross-examined by defense lawyers about the unrecorded conversations on Nov. 15, 1979, which were made from a hotel suite at a casino resort in Atlantic City, N.J.

The telephone calls were between former Richmond businessman John R. Stowe and Melvin Weinberg, an FBI informant and convicted con man, according to evidence in the case. During those conversations, Amoroso has testified, Weinberg first told Stowe, a longtime friend of Jenrette, that he represented Arabs who would pay $100,000 for private immigration legislation.

Amoroso has also testified that Stowe said later that same day in another unrecorded telephone conversation with Weinberg that he had talked to Jenrette and that the congressman had agreed to a meeting on Dec. 4, 1979.

The government has contended that the telephone calls were the beginning of a criminal conspiracy in which Jenrette and co-defendant Stowe planned to take payoffs from the fictitious Arabs in exchange for Jenrette's promise to introduce the legislation.

Amoroso testified yesterday that he must have been "totally confused" when he was questioned by the defense last week about the recording equipment. He told the jury yesterday that while he did not have the usual equipment available to tape telephone calls, other recording devices were available in the Atlantic City hotel room at the time.

Amoroso testified, however, that he didn't think the two conversations were "important" and "never thought" to record them. He has previously testified that he was concerned about security in the hotel suite, where he and Weinberg were hosting a hospitality reception for local politicians.

During lengthy questioning at the trial, Amoroso has repeatedly said that no notes or records were taken during the Abscam investigation -- other than the tapes -- and that the defense would have to rely on his memory, his "word," and the tapes made during the Abscam sting.

Amoroso agreed with Stowe's lawyer, Murray Janus, under cross-examination yesterday, however, that both sides in the case would have been incorrect if they had relied on his word about the availability of the recording equipment based on his testimony last week.

In another development, Weinberg, who continued his testimony yesterday, disclosed that he has a book of telephone numbers, which he possessed during the Abscam investigation, that has not been seen by defense lawyers. He made the statement, outside the presence of the jury during questioning by Judge John Garret Penn about whether he had any records or notes on the case. Penn ordered the government to produce the telephone book for examination before the start of today's court proceedings.

During that hearing, Weinberg also told Penn that he sometimes had difficulty remembering what people were talking about when he got phone calls in connection with the Abscam investigation and, he explained, "That's why I was doing a lot of stuttering on the phone."

Weinberg also said yesterday that at the outset of the investigation he was told by the FBI to record only "important" discussions because the bureau was short on cassette tapes. Weinberg said he even bought his own tapes and a recorder at one point.

One example of apparent confusion came earlier in the day when the jury heard a taped conversation between Stowe and Weinberg in which Stowe said that Jenrette had talked to Sen. Strom Thurmond (R-S.C.) and claimed Thurmond would introduce a private immigration bill for the Arabs for money. In testimony last week Thurmond denied that he ever spoke to Jenrette about such a deal.

During that Jan. 29, 1980, coversation. Weinberg said, "That's the guy in charge of the . . . Democratic Party, ain't it?" Weinberg said yesterday he thought Stowe was talking about Robert Strauss, who at the time was running President Carter's reelection campaign.

The government's tape-recorded evidence showed that Stowe picked up $50,000 from Amoroso, who was posing as "Tony DeVito," a representative of the Arabs, at a house in Northwest Washington and said he was delivering it to Jenrette's office. Stowe has said on government tapes that he received $10,000 of that money.

There is also tape recorded evidence that Jenrette later sought a $150,000 loan from the fictitious Arabs and then proposed the immigration deal involving Thurmond, which Jenrette said on the tapes would cost $125,000. The government contends Jenrette intended to pocket that money for himself.