It could be months before federal workers with post-1956 military service find out from Uncle Sam how much money they must put into the civilian federal retirement system to get larger lifetime pensions based on the time they spent in uniform.

Approximately 1.4 million of the government's 2.7 million civilian federal workers are military veterans. There are 144,000 military retirees in civilian federal jobs. Many of them have military service after January 1957, when the military went under Social Security.

Feds with military service can count their time in uniform toward civilian retirement, thereby increasing their annuities. But until recently they lost that military service time--and had their civilian pensions cut back--when they became 62 if they were eligible for Social Security. That take-back was called Catch 62.

Last year Congress voted to eliminate the Catch 62 reduction for persons already retired from government. Annuities that have been cut by Catch 62 will be recomputed and restored to the higher level.

Congress also gave present U.S. workers with post-1956 military service the option of buying their way out of the Catch 62 annuity reduction. It said they would not be subject to the reduction if they would pay into the civil service fund for the time they spent in service after January 1957. In effect, they must contribute 7 percent of their military earnings to the fund, just as if they had been civilian federal employes.

But instead of allowing employes to estimate their military earnings, Congress said the records must be checked for each individual.

The problem for most people is finding out how much they earned in the military to determine how much they must pay into the civilian fund to avoid the Catch 62 reduction. Since most of us did not keep our military pay stubs, service earnings are hard to reconstruct.

Earlier this year the government told employes with military service to write to the finance center of their service branch for their pay records.

Thousands of people have written the centers--Indianapolis for the Army; Denver for the Air Force; Cleveland for the Navy; Kansas City for the Marines and Topeka for the Coast Guard--only to be advised that military pay records are destroyed after 6 1/2 years.

The Department of Defense has advised finance centers to make a careful serach of any available records and to estimate as closely as possible each individual's earnings. Workers will then be advised how much they must contribute to the civilian retirement fund to protect their military service credit when they retire.

In many instances, the military will have to get service personnel data (time of service, promotions, etc.) from the General Services Administration's National Service Records Center.

The point is that it will be some time before each and every case can be handled. There are a lot of records to go through and, wonderful as computers are, a lot of this must be done by hand.

Those who are anxious to get a quick answer to what they made (and what they owe) can speed things along if they send the appropriate military finance center information such as their exact dates of service, citations for promotions, ranks held and the like.

This is another case where Congress changes a law but leaves it to the career federal work force to make the changes, within the guidelines laid down by the Congress. Pulling and reviewing the records will be a nightmarish chore, and the inevitable delays will be blamed on bureaucrats, even though they are just doing what Congress told them to do, in the manner Congress told them to do it.