A former AG who also put in time at DOC, DOD, and HEW reported to Congress yesterday on his latest governmental responsibility: technical problems in the RSNT of the LOS.

Elliot L. Richardson emerged from what he described as "several weeks of getting up to speed" on his subject matter to give the House Merchang Marine and Fisheries COmmittee his views on the Revised Single Negotiating Text (RSNT) of the U.N. Conference on the Law of the Sea (LOS).

Richardson, the only man in history to hold four Cabinet posts, was named U.S. ambassador to the conference by President Carter las tmonth. A veteran member of the "statesmen's pool," he has received nine major presidential appointments in a federal career spanning four administrations.

The Law of the Sea Conference opened in Caracas in 1974. Its sixth plenary session begins in New York on May 1, and Richardson said yesterday that there is "some chance" a final treaty will emerge from that session.

Richardson, the first Republican to receive a major appointment in the Carter administration, said yesterday he still does not know how he was picked for the job.

"There have been about five people in this area who assure me that they were responsible for it," he said. "All I know is that I got a call out of the blue from Cy Vance asking me to take it on."

Richardson and Scretary of State Cyrus R. Vance last worked together in 1969, when Richardson was under secretary of state and Vance a negotiator at the Paris peace talks.

The ambassador-at-large-designate, who successively ran the Departments of Health, Education and Welfare, Defense, Justice and Commerce (with a brief respite as ambassador to England) in the Nixon and Ford administrations, said he had planned to make a leisurely lecture tour this year.

"But then this came along. I gues I feel an irresistible impulse to take on whatever is the most troublesome mission anybody offers me."

The nomination of the Massachussetts Republican angered House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.), who reportedly told Carter that Richardson would use the post as a springboard to a race for governor in his home estate.

"I didn't call Tip about that," Richardson said yesterday. "But I wanted to tell him, look - if I decide to run for governor, I don't need another line on my resume, for God's sake.

In a 25th reunion book compiled by his Harvard class in 1966, Richardson wrote, "In matters of employment, I have continued a nomadic behavior that I can keep a steady job."

In the 11 years since he has held nine positions.

Richardson said he was moderately well-prepared for Law of the Sea Conference by tangential contacts with the issue in his Cabinet posts. He said "very intense reading" would probably give him adequate command of the field before the New York session.

In yesterday's hearing, the new ambassador spoke with precision and assurance about such technical details as "site-specific legislation" and the "interim regime concept."

Twenty-six of the committe's 41 members showed up for theh session - an unusually large turnout, accordig to the committee staff.

Most members used their allotted question time to praise Richardson's nomination and to express confidence in his success at the conference.

"I'm just convinced that you are so smooth and so diplomatic . . . that you'll bring back a treaty the Senate will approve," said Rep. John B. Breaux (D-La.).

Later, as a government chauffeur drove him to lunch at the Sans Souci, the hero of the "Saturday Night Massacre" reflected on his new $57,500-per-year federal job.

"You have to negotiate both domestically and internationally on these sea issues," he said. "This thing may be the most difficult job they've ever given me."