It is a recurrent playlet, one of the familiar set pieces of our political system: the last days in office of a President serving out a term.

The mood typically is piognant. "It has been a little sad for me the last few days," Lyndon Johnson confessed as he said good-byes at a reception given him by the Senate in January, 1969 - and Gerald Ford must have thought, if not said,the same this teek.

Outgoing Presidents characteristically indulge in a little anticipatory history-writing. "We have averted World War III up to now." Harry Truman said in a farewell address in January, 1953, while Johnson told Congress in 1969 that in the five preceding years it - and he - had "written a record that I think has never been matched in all of our 188 years."

There is also a certain kind of government business that seems generally to get done - in some cases can only get done - in the waning days of gn administration.

Leaving office, Presidents can be sometimes plainspoken in a way they could not before. They also can - and often do - reach one last time into the federal cookie jar, not for themselves, but others. Or else they keep someone else from reaching.

Thus, one of the last things Truman did was issue an executive order setting aside "The submerged lands of the continental shelf as a naval petroleum reserve," protecting them from private exploitation - an order long since rescinded.

Truman's final official act in office was to sign a letter to the then-president of the American Federation of Government Employees, the largest federal employee union. The ostensible occasion was the 70th anniversary of the federal merit system.

The real reason for the letter was McCarthyium, the accusations by the late Sen. Joseph R. McCarthy (R.Wis.) that the civil service was disloyal. Truman said that, to the contrary, it was a "great asset," and added his "fervent hope . . . that recent reckless attacks which can destroy that great asset will subside."

Thus, departing Presidents can be defiant. They can also be acqulescent. One of the routine messages Truman sent Congress as he wasd leaving was the first volume of a report from a presidential commission on the Health Needs of the Nation. Truman agreed on the message theat it might be wise to give up for a while an idea he had pressed for years - "a national helth insurance system." That remains an issue today.

So does another subject on which Truman and Johnson both sent recommendations to Congress as they were departing: federal pay.

Truman urged that future Presidents Vice Presidents, and Speakers of the House be granted larger tax deductions for the expenses involved in their offices.

Johnson, in departing, urged general compensation increases in the upper reaches of the government, and now Ford has done the same.

Sometimes departing Presidents send Congress large numbers of nominations for federal jobs. Eisenhower nominated 1,160 persons for postmasterships 10 days before leaving office, plus one member each of the Interstate Commerce Commission and Civil Aeronautics Board.

Presidents use their last days to unburden themselves in other ways. Eisenhower waited until his farewell address to the nation to warn of the "military-industrial complex." On a smaller scale, Johnson backed off a false claim he had once made, that he had an ancestor who had fought at the Alamo. He had meant "the Alamo Hotel in Eagle Pass, Tex.," he told the press corps.

Presidents also leave little time bombs for their successors sometimes. Johnson ordered five Southern school districts to desegregate - effective three weeks after Richard Nixon took office.

Ford made a number of decisions President Carter will now have to accept or expressly reverse - the licensing of two deepwater "superports" in the Gulf of Mexico for oil supertankers, for example, and the decision announced Wednesday to guarantee $730 million in loans to help General Dynamics Corp. build seven tankers to ship liquefied natural gas. The decisions will draw fire. Fordwill not be here to feel it.