IN 1988, when the last postal rate increase was announced, postal executives expected to be flooded with demands for 25-cent stamps, the new first-class rate.

Customers did storm post officies, but what they demanded were three-cent stamps, the so-called "make-up" rate between the old 22-cent first-class rate and the new 25-cent one. As a result of that surprise, postal executives have made it a priority to have plenty of the make-up stamps on hand for next year's rate change.

Next Friday the Postal Service will try again, rolling out another of what it hopes will become a well-used rate-change stamp for 1991. The stamp, to be released in Syracuse, N.Y., is a red five-cent stamp that Americans will see a lot of, if the independent Postal Rate commission agrees to the service's request to boost the cost of a first-class stamp to 30 cents.

That would add 5 cents to the cost of sending a first-class letter and it would make the new stamp, which bears the image of a circus wagon, an ideal companion for the millions of 25-cent stamps that are certain to be in customers' hands when the rate change becomes effective early next year.

Earlier this year the Postal Service issued another 5-cent stamp in sheet form. It carried the image of Luis Munoz Marin, Puerto Rico's first popularly elected governor. Next Friday's release will be a new 5-cent stamp in rolls, or coils, of 500 and 3,000. That will make the circus wagon stamp perfect for bulk mail users, who have made best sellers of the other coil stamps in the service's long-running transportation series.

The new stamp is the first designed by Susan Sanford of Washington. It is also the first in the transportation series to carry the "05" denomination. These denominations are designed to help mail clerks distinguish cheap stamps from high-valued ones with the same numerals, in this case $5 stamps.

Sanford's wagon will be printed by the Bureau of Engraving and Printing by the intaglio process. Her stamp, which was engraved by the bureau's Gary Chaconas and Gary Slaght, is described as a composite of the early wagons used by circuses that traveled across the country in the early 1900s.

By releasing the stamp this summer, there should be millions at post offices before the rate change becomes effective. But what if the rate commission nixes the Postal Service's request for a five-cent increase and calls for a four-cent increase? The service has been a plan for that, too.

It has printed millions of make-up stamps that won't carry a denomination, but will be sold for whatever the difference is between the current 25-cent stamp and the new first-class rate.

Officials have yet to announce the design of the stamp, but the development is a logical outgrowth of the series of so-called "letter stamps" that the service began issuing in 1978. Those are first-class stamps that carry a letter instead of a specific value. They are printed well in advance of any rate change and then are offered to customers at whatever the commission decides the first-class rate should be.

Next year's first-class letter stamp officially will be known as the "F stamp" but postal officals have said they will call it "the flower stamp" after its tulip design.

THE U.S. MINT has acknowledged a major error in proof sets of its 1990 coins. The highly polished sets of coins are manufactured at the mint's San Francisco facility. Unlike the pennies in general circulation, the sets' pennies were supposed to bear a small "S" indicating they were struck in San Francisco.

But officials last month discovered that one of the hundreds of dies that should have been shipped to the Philadelphia Mint, where most pennies are made, was shipped to San Francisco by mistake. The die, which did not carry a mint mark, apparently was used to make "approximately 3,700" proof-quality coins.

Mint officials discovered the error when a customer called. That triggered an investigation that has led Mint officials to conclude 3,555 of the S-less pennies were shipped from San Francisco. Another 145 were discovered in coin sets the Mint still had in its vaults, a spokesman said. They will be destroyed.

That should only enhance the value of those S-less pennies. It seems certain to have made the Mint's $11 proof sets one of the best coin values ever offered by Uncle Sam.

Bill McAllister is a member of The Washington Post national staff.