The man who came to dinner last Thursday night was none other than Ronald Reagan -- making his first social appearance outside the White House since the assassination attempt seven weeks ago -- but the man who gave the dinner was Thomas V. Jones, chairman and the chief executive officer of Northrop Corp., who pleaded guilty in 1974 to violating federal campaign contribution laws.

Both the White House press office and Nancy Reagan's press office said they did not know who Tom Jones was other than that he was the host of the party at the George Town Club, but a Northrop spokesman later confirmed that it was indeed the same Tom Jones of Northrop. The White House said it did not have the guest list for the party.

Tight security pervaded the arrival and departure of the president and Mrs. Reagan, complete with Secret Service sharpshooters poised on rooftops overhead. One dinner guest said later, "I thought there was a car wreck what with all the cops outside." The dinner party was in honor of the Reagans' good friend Jack Wrather, who was in town to host a premier of his movie "The Legend of the Lone Ranger." The Wrathers, Joneses and Reagans are all friends, according to the Northrop spokesman. Jones' secretary said Mrs. Jones and Mrs. Wrather drew up the guest list.

The adventures of Tom Jones began in 1974 when Jones pleaded guilty to felony charges of making illegal donations of $150,000 to former president Richard Nixon's 1972 reelection campaign. Both the firm and Jones were fined $5,000 each. The Watergate special prosecutor's office brought the case under a 1940 statute prohibiting political contributions by government contractors.According to information filed in court against Northrop and Jones, the money was contributed to the Nixon reelection campaign by funneling corporate funds through a European consultant of Northrop's. Prosecutors also charged that Northrop backdated certain documents to cover the contributions and that the entire illegal transaction was aided and abetted by Jones.

In another legal action, in 1975, the Securities and Exchange Commission filed a complaint alleging among other things that Northrop and Jones had maintained a secret corporate fund in excess of $476,000, a substantial portion of which had been used for unlawful political contributions and related expenses. Jones and Northrop consented to the entry of an order enjoining them from future violations without admitting or denying the allegations of the complaint.

Northrop is still a top defense contractor. In 1980, Northrop was the country's 11th-largest defense contractor with $1.2 billion in defense contracts, according to a Defense Department spokesman, including two MX missile contracts, and it is currently producing parts of the F18 Navy fighter plane.

The two congressmen on the guest list were Rep. G. V. (Sonny) Montgomery (D-Miss.), a member of the House Armed Services Committee, and Rep. Charles Wilson (D-Tex.), on the House Appropriations Committee.

Montgomery said Tuesday he didn't know either the Wrathers or the Joneses but Sen. Lloyd Bentsen (D-Tex.) and his wife were friends of the Wrathers and they recommended he be invited. "Charlie Wilson and I are both Democrats and both bachelors. There were single ladies there and that was why we were invited," Montgomery said. "It was a friendship party; there was no talk of defense contracting. I'm on the Armed Services Committee and I would have heard any talk about it."

Wilson, who says he was there to "even up the count," agreed that the dinner was "all of Reagan's old friends . . . reminiscing about old Hollywood days. It was the first time some of them had been in Washington since the inauguration and they were complaining all they ever got to see was the inside of the White House and the Madison Hotel."

Aside from Montgomery and Wilson, the guest list was stellar Republican, with such Reagan friends as the Alfred Bloomingdales, Attorney General William French Smith and Wyatt and Nancy Dickerson. As one of Jones' secretaries put it, "They've all been out here a long time and all have known each other for years."