A record number of federal workers are expected to retire between now and mid-January to take advantage of a 7.7 percent pension-booster benefit that will be wiped out permanently later next month.

If government estimates of 70,000 retirements hold up, it would be the equivalent of a giant cabinet-level department such as Interior or Transportation disappearing almost overnight just before President-elect Reagan takes office.

President Carter signed legislation on Friday that eliminates the so-called look-back feature of current federal retirement law. The look-back feature enables U.S. workers to retire and get the benefit of the most recent cost-of-living (COL) raise granted already-retired civil servants. Federal, postal and military retireees get regular COL increases every six months, in March and September. The most recent COL raise for retirees was a record-high 7.7 percent increase that went into effect last Sept. 1.

Congress originally intended to elminate the look-back feature effective as soon as the bill was signed into law. But the November elections forced more than a hundred Senate and House members out of office. Their terms will expire in early January. To let the lame ducks get benefit of this last COL look-back, Senate and House budget conferees agreed to extend eligibility for it 45 days after the bill was signed by the president.

In its haste to adjourn and ensure the retirement bonus for lame-duck members, Congress completed work on the bill and sent it to the White House Friday. Carter signed it on the same day -- Dec. 5. The 45-day eligibility countdown started as soon as he signed the bill.

Another feature of the bill, which is effective immediately, eliminates a look-forward benefit government workers once enjoyed. It allowed them not only to get all of the COL that took place before they retired (the so-called look-back feature) but also to get the full amount of the COL raise that occurred after they retired, even if they quit only one day before the six-month inflation adjustment went into effect.

As of now, persons retiring will have their initial COL raise prorated on the basis of how long they have been retired. In other words if you quit one month before the COL goes into effect, you get one-sixth of it.

Both the look-forward legislative change (effective immediately) and the look-back change, effective 45 days after signature by the president, were part of the budget reconciliation bill that Congress rushed through in its final days. Originally Congress intended to eliminate one of the two COL raises that U.S. retirees get.

In their biggest muscle-flexing exercise as a voter bloc, federal and postal workers and particularly retirees, persuaded nervous politicians not to tamper with the twice yearly inflation adjustments. In April this column asked U.S. workers and retirees if they felt strongly enough about the COL issue to vote against anybody who favored changing it. More than 65,000 people responded -- it was the biggest flood of mail The Washington Post ever received on a single issue. And that message wasn't lost on the Congress.

This is the situation now: The look-ahead benefit is gone. The look-back feature will be eliminated in mid-January. The twice-yearly COL adjustments are safe, for now, and presumably for the next four years if President-elect Reagan keeps his promise.