Federal workers and retirees who served in the military after Jan. 1, 1957 can now get both Social Security coverage and federal pension credit for that time if they make contributions to the civil service retirement fund for the time they were in uniform.

Until recently, many federal workers whose annuities were based on combined military and federal service time lost the military service credit (and took a corresponding annuity reduction) when they reached age 62 and became eligible for Social Security. That was called Catch-62.

Last year Congress enacted legislation to make it possible for such individuals to get credit under both systems. To do it, they must pay into the civilian federal retirement fund an amount equal to 7 percent of their military base pay to avoid losing military service credit at age 62.

Details of the extremely complicated payback procedure (it is 12 typewritten pages, plus charts) are being provided to federal agencies in a special Federal Personnel Manual Letter. Advance editions of the FPM Letter (Number 831-77) have gone to headquarters personnel offices. Field installations should have it later this month.

The option to avoid the Catch-62 annuity reduction is important and far-reaching, because nearly half of all federal workers have some post-1956 military service they may want to credit to their civilian service time to boost their annuities.

Individuals who wish to avoid the age 62 annuity cutback can do so by making contributions to the civil service retirement fund via payroll deduction. The payback can be made without any interest charge through Oct. 1, 1984, or two years after the employe was hired, whichever is later. After the deadline, an individual can still make the payback and avoid Catch-62, but will have to pay interest.

If you are in government now, check with your personnel office for details. Retirees will be contacted by the Office of Personnel Management advising them of the procedure.