More than a million U.S. workers with post-1956 military service face a possible cutback in their federal retirement benefits if they have combined their military-civilian service time for retirement purposes, or plan to.

Employes who do that face a shock on their 62nd birthday when they become eligible for social security.

At that time they lose credit for that portion of their military service that was covered by social security, and the result is that their civilian annuities are recomputed and reduced. It is called Catch 62.

Last year Congress passed legislation to permit employes to avoid that catch.

In effect the law allows current U.S. workers (who have served in the military from 1957 to present) to contribute 7 percent of their military earnings into the civil service retirement fund. This insures that they won't be hit by Catch 62.

Persons who were retired on the date of enactment Sept. 8, 1982, will not be subject to the offset. And those who have already had their civilian annuities reduced by Catch 62 will, eventually, have their annuities recomputed under a new formula that will mean an increase for most of them. The readjustment will be retroactive to last Oct. 1.

Current employes need to find out (from their service branch) how much money they made in the military from 1957 onward. Once they establish that amount they have until October 1984, or two years from the date they entered the government, to put that 7 percent contribution into the civil service retirement fund. They can make the payments later but they will be subject to interest ranging from 6 to 8 percent.

Once they find out how much they owe, payments can be made through the federal agency, using a redeposit form SF 2803. Because of the difficulty in locating and/or reconstructing military pay records, military finance centers are slow -- with the exception of the Navy -- in getting the information.

This is where you should write to request a statement of post-1956 military earnings:

Army Finance and Accounting Center, Department 88, Indianapolis, Ind. 46249.

Navy Finance Center, attention 43C, Celebreeze Federal Building, Cleveland, Ohio 44199.

Air Force Accounting and Finance Center/ASC, Denver, Colo. 80279.

Marine Corps Finance Center, Kansas City, Mo. 64197.

Coast Guard Settlement and Records Center, 444 S.E. Quincy St., Topeka, Kan. 6683.

Public Health Service, Parklawn Building, Room 4-34, 5600 Fishers La., Rockville, Md. 20857.

National Oceanic and Atmospheric Administration, NOAA, Rockville, Md. 20852.