The Postal Service has decided to play hardball with its potential competitors, the private delivery services that say they can do it cheaper and quicker.

In a rule published in the Aug. 22 Federal Register (page 56054) the service in effect barred use of private letterboxes for anything not bearing a postage stamp. Letterboxes are the locked boxes in apartment houses and the free-standing mail receptables with metal flags found in front of houses in rural and suburban areas.

A law passed in 1934, according to a Postal Service lawyer, made it a criminal act to deposit anything not sent through the U.S. mails in such a receptacle, which officially is titled as "authorized depository." A small slot that goes directly into the recipient's house is something else.

That Depression-age law came out of something similar to today's situation. In those days, utility companies started to have the men who read gas and electric meters at each home also deliver the monthly bill.

It was done to save postage. The Post Office Department, however, complained to Congress that the loss of revenue was endangering service for everyone. The result was the present law.

Today the growing threat of alternate private delivery systems -- particularly for parcels, books, magazines, newspapers and records -- is considered just as real by the Postal Service. The new systems, according to a service official, have not made a major attempt to use mail boxes, but "there have been a number of instances" that present "a minor problem."

The new rule, he added, was intended more to instruct service employes on how to handle "this minor problem" and also to let everyone know about the law -- particularly the new delivery systems and anyone interested in starting one.

Under the new rule, a letter carrier who finds unstamped, privately delivered material in mail boxes at some but not all addresses along his or her route is required to pick them up and return the to a Postal Service delivery unit. There postage will be assessed and a request for payment made to the delivery firm responsible, if the delivery firm cannot be found or refuses to pay, the material will be returned to the publisher or manufacturer -- postage due.

If the nonpostage-bearing material has been delivered to all addresses on a route, the letter carrier need pick up only two or more as a sample. The postage will then be computed and assessed against the delivery system or the publisher or manufacturer.

The Postal Service refused to allow a waiver for first violations, saying "a free 'first bite' would only introduce unnecessary complications."