Uncle Sam could collect millions, maybe billions, of dollars from federal workers with post-1956 military service (who are, no doubt, anxious to pay up) if the Defense Department could only figure out how much to bill them.

The money would help the federal retirement system right now. And it would buy future pension protection for 1.5 million feds who have served in the military since January 1957.

Military service time can be credited toward civil service retirement. But if any of that military time was after January 1957 and if the retiree becomes eligible for Social Security, the civil service annuity is recomputed at age 62 and reduced to take away military service time. January 1957 is the magic cutoff because that is when the military went under Social Security.

Because it hit on employes' 62nd birthdays, the annuity cutback was called Catch 62. Last year Congress decided to do something about it.

It has been nine months since passage of the law allowing feds who wish to avoid the age 62 annuity reduction to do so. Congress said employes who are military veterans could pay their agencies the equivalent of 7 percent of their post-1956 military earnings. That would give them civil service retirement credit for that time and protect them from any future annuity reduction.

The money employes "pay back" into the government would go into the civil service fund and be used to pay benefits.

Congress thought it could clear up everything by passing a law to correct an earlier law. It wasn't so easy.

Congress said, in effect, just tell your agency how much you made in the military, pay 7 percent of it and you are home free! But it hasn't been that simple, because few people keep their military pay records dating back to the late 1950s. And the government doesn't accept your word of honor as to rank, dates of service and pay rate.

Since most people do not know how much they earned in the military--or can't prove it if they think they know--people have been writing the finance centers of the Army (Indianapolis); Navy (Cleveland); Air Force (Denver); Marine Corps (Kansas City) and Coast Guard (Topeka, Kan.) for their records.

So many have been writing, the Federal Times reports, that finance centers have a backlog of 20,000 requests. Many people who have gotten "answers" from the finance centers say they didn't help much.

One military finance office has been writing Catch-62 avoidees that all pay records are shredded after five years. Others say that some military service records were destroyed in a fire several years ago at the St. Louis Federal Records Center. Most people have been told to wait until Defense devises a system to estimate how much people made during those years. That could be months.

Meantime there is a deadline (October 1984) for most people to make the payments to the retirement system or be charged interest.

Most officials think the government will have the data to employes in time for them to beat the deadline. But it has already been nine months and little has happened.

This is a classic case of Congress waving its magic wand, but leaving the troops in the trenches--civilian and military--to make the system work. Relief is on the way. We just don't know when.