If you are one of a special group of 800,000 persons who worked for the U.S. Postal Service between May 1974 and May 1978, be advised: The check is almost in the mail.

The USPS is preparing to crank out $400 million by late spring to many veterans of the service during the 1974-78 period.

Checks to individuals will range from $6.30 to $759.20, depending on how much overtime USPS figures it owes them for those years.

The payout is the result of a 1982 order from the U.S. District Court. It said, in effect, that the mail-moving corporation had not complied with the Fair Labor Standards Act during that period. The law, administered by the Labor Department, required that employes working or involved in job training be paid overtime. USPS estimates that as many as 800,000 workers, some of them dead or long gone from the service, are owed the money.

Postal officials say that persons due the back pay who are still on the job will get special checks at the office. It hopes to deliver them before the summer.

Ex-postal workers, retirees or the survivors of those due the money will be notified by letter of the impending payments. The purpose of the letters is to make sure the USPS has correct addresses. Once those addresses are confirmed, USPS says, the checks will be in the mail.