Helen Petersberger of Northwest obviously could never have been a Boy Scout. Still, she believes in the famed Scouts motto about being prepared. So when first-class postage rates jumped to 18 cents last March 15, Helen rushed to the post office and bought a bucketful of those purple 18-cent stamps that bear the profile of an eagle, wings outspread, and are marked with the letter "B."

The B stamps were issued, you'll recall, because there was uncertainty right up until March 15 about how much first-class postage would cost after that date.

Rather than issue thousands of 17-cent stamps, and then learn that an 18-cent rate had been authorized for first-class mail, the Postal Service hedged its bets with Bs. They were sold for 18 cents, as it turned out, but they weren't marked as such.

Which was exactly why Helen Petersberger ran into trouble last week as she tried to mail a letter to France that bore two Bs. The clerk said they weren't usable on foreign mail, including letters bound for our closest neighbors, Canada and Mexico.

Helen protested. "My reasoning was that if you pay 18 cents for a stamp, you ought to be able to use it," she said. But she got about as far with her complaint as baseball managers usually do with umpires.

Why so? According to George Conrad, public information officer for the Washington office, the decision to bar B stamps from foreign mail stems from the experience the postal people had in 1975 with the B's ancestor, the A.

"We found that some foreign pieces came back in 1975 because postal authorities in other countries were not familiar with the 'A,'" said Conrad.

"The U.S. is a party to the universal postal union convention. One of its requirements is that stamps shall bear the name of the originating country in Roman letters and the postage value in Arabic figures. The B doesn't, so that's why."

But couldn't the U.S. simply cable other postal convention countries and notify them of the special circumstances that surrounded the issuance of the B? "We did," Conrad said, "And I'm sure that's why there's been a very small rate of return on letters with Bs on them."

The irony of the Petersberger story, says Conrad, is that "she made the mistake of going to a post office. If she'd just dropped it into a mailbox, the odds are very great that there would have been no problem as long as the total of postage was right.

"But don't get the wrong idea. We can't promise that her letter would have gone through. The best way to make sure of getting a foreign letter where it's going is to follow all the rules."

ERRATA, MINE AND OTHERS: On July 13, I reported that an off-duty Washington policeman named Alvah D. Adams had heartwarmingly helped an elderly man change a tire on a Bethesda street. As soon as I saw a letter a week later with "Mrs. Alvah David Allen" printed on the back flap, I got that sinking feeling.

Sure enough, Dave and Vivi Allen of Bethesda were writing to say that they had been delighted to read of the exploits of their son, Alvah Jr. "Adams is a good name," they noted, "but we like Allen."

Between bites of crow, let me offer this feeble defense:

Before printing the item, I checked the name of the helpful policeman with the 7th District dispatcher. We spent so much time on the Alvah that, between us, we must have fumbled the Allen. Apologies to all Allens concerned.

Meanwhile, Doug Schindler of Fairfax snared an error of attempted overkill in last Monday's Washington Business section.

A photo caption described an electronics company chairman posing in front of "one of the firm's antennaes [emphasis mine]." As Doug points out, if one plural isn't enough, why not try two?

And it isn't every reader who will correct a bad gaffe with a good poem. But such was the case with Owen J. Remington, of Lancaster, Va.

In a Post story last week about whooping cranes, Owen read this sentence: "Then Grandy added the coup d'grace."

His reply:

"De before consonant/

"'D' before vowel

"To do otherwise/

"Makes the Frenchman howl."