The nation's 600,000 postal workers will get a little somthing extra in their paycheck envelope this week: a note from the boss saying that it is illegal to strike Uncle Sam and that nobody who gets involved in a walkout can keep on walking.

Purpose of the insert, planned for this final paycheck before the Monday midnight expiration of the contract between the Postal Service and its heavily unionized work force, is to prevent (officials hope) a strike if no agreement is reached between the Postal Service and the four unions now yelling at each other across the bargaining table.

The two unions representing about 400,000 postal workers -- the National Association of Letter Carriers and American Postal Workers Union -- both have membership mandates they say no-contract, no-work.

Even if tentative agreement is reached before Monday it could be weeks before union members can vote, by mail, whether to accept or reject. Postmaster General William Bogler, himself an up-from-the-ranks mailman, wants to be sure the mails keep moving for the health of the government-owned corporation and the nation's economy.

On a typical day, 336 million items, from Social Security checks to diamonds, go through the U.S. mail system. A breakdown anywhere like New York City, which handles more mail than the British Isles, could cause a logjam delaying everything from rent checks to veterans payments.

Bogler's memo to employes will say the Postal Service is "committed" to an on-time contract (unions say he delayed bargaining for nearly seven weeks) and that "it is important that all postal employes recognize that there are orderly and peaceful procedures provided . . . for resolving an impasse should one arise."

The paycheck insert will advise that strikes are illegal (punishable by dismissal, a $1,000 fine and/or a year and a day in jail) and that management will be much tougher than it was in August 1978 when workers in New Jersey and California staged wildcat walkouts to protest a tentative new contract their leaders had accepted.

More than 200 employes were removed from their jobs. While most were eventually reinstated, some are still unemployed and can never again work for any government agency. Bolger's note says "you should be aware that any employe who in any manner participates in a strike or other illegal job action will be discharged."

Members of the National Rural Letter Carriers Association, also in the bargaining talks, got their monthly magazines yesterday with a plea from their leaders to cool it. NRLCA made clear it will have no part of a strike.

While grateful for the NRLCA stance, postal officials realize that worker militancy is concentrated in big cities and bulk mail centers, the giant mail-moving factories where rural carriers are as rare as polar bears in the Congo.

There is a good chance that the Bolger insert will futher inflame hit-the-bricks advocates in Philadelphia, San Francisco, Chicago and, of course, New York City. The first postal strike, which idled more than 150,000 workers, began there on St. Patrick's Day, 1970.

The leaders of the militant New York area local during that strike were Vincent Sombrotto, representing workers in the Letter Carriers union, and Morris Biller, representing employes of the APWU. The regional postal official on the spot was one Bill Bolger.

Today Bogler sits in Benjamin Franklin's old seat. Sombrotto is the national president of the Letter Carriers. Biller heads the APWU. All three got where they are today by telling their constituents "never again!"

New Yorkers Biller and Sombrotto turned their "soft" predecessors out, and say the won't take any guff or an unsatisfactory contract from the Postal Service. Bolger is also a "never again" type, becoming postmaster general because he is a tough insider who will not back down from a strike, or forgive any employe who participates. This is going to be a long, macho week for people who move the mail and their customers.