WHEN SIR Rowland Hill, a British education and colonial reformer, turned his attention to his country's chaotic mail system 153 years ago, he offered a revolutionary idea. A cheap, uniform postal rate, Hill argued, would produce a surge in mail volume and increase commerce and literacy in the process.

Then, in what Britain's Royal Mail service has described as "almost an afterthought" he suggested that "a gummed printed label" could be affixed to each letter to indicate the postage had been prepaid by the sender. Until then the postage on most letters had to be paid by the recipient -- and at widely varying rates.

The British government accepted Sir Rowland's idea and on May 6, 1840, it officially introduced the world's first postage stamp. By the end of the year more than 60 million of the black one-cent stamps bearing a profile of Queen Victoria had been produced, and what the Royal Mail calls the world's most popular hobby had been born.

This year countries around the world are celebrating the 150th anniversary of Sir Rowland's "afterthought," issuing scores of stamps to mark the birth of the stamp known simply as "the Penny Black." The stamp and its two-cent blue companion, the "Two-Penny Blue" are widely available, often being discounted by dealers below their catalogue value of $2,000 for an unused Penny Black and $6,000 for the Blue in mint condition. Used copies are much cheaper, listing at $140 in the current Scott's Stamp catalogue for the Penny and $300 for the Blue.

If a young collector can't afford one of the Penny Blacks, then this is the year to get a high-quality reproduction. Samoa, Ascension, Vanuatu, Swaziland, Barbados, Pitcairn Islands, the Seychelles and the Marshall Islands are a few of the countries that have weighed in with stamps that reproduce the Penny and the Blue.

Britain was the first to mark the anniversary, releasing in January a set of five stamps called the "Two Queens" set. Each stamp bears a profile of Elizabeth II set against the Penny Black's portrait of Victoria. An international stamp show, Stamp World London '90, followed last month, bringing collectors and dealers from around the world to what amounted to a gigantic birthday party for the first postage stamps.

One of the most interesting of the non-British stamps came from the six-year-old Republic of the Marshall Islands and its American agents, the Unicover Corp. of Cheyenne, Wyo. They hired Jeffery Matthews, the same Londonartist who created Great Britain's "Two Queens" stamps, to design their stamps.

For the Marshalls, Matthews created a booklet of six stamps and a souvenir sheet in honor of the Penny Black. What makes the set educational is that Matthews's six stamps show various proposed designs from which the Penny Black evolved. The souvenir sheet features a portrait of Charles Heath who, along with his son Frederick, engraved the Penny Black.

In addition, the Marshall Islands' set also include a stamp with a large reproduction of the Penny Black and the City of London medal, from which the portrait of Victoria was used as the model for the stamp.

The booklet, with its six 25-cent stamps and a souvenir sheet with a $1 stamp, contains a brief history of the Penny Black and each stamp. The booklets sell for $2.50 each from the Philatelic Center of the Marshall Islands, One Unicover Center, Cheyenne, WY 82008-0021.

MEMBERS OF Congress long have insisted that anyone who wants one of the country's new coins should be able to buy one directly from the U.S. Mint. Now, to the delight of the nation's coin dealers and many collectors, that may be about to change.

Late next month, Mint Director Donna Pope will announce the first limits on the number of proof sets of 1990 gold and silver American Eagles her agency will sell. The decision to place a ceiling on proof production represents a sharp break with the past, but it comes in the face of declining sales of the highly profitable proof sets.

Until now, the Mint has had an open-ended sales policy, producing all the proof coin sets it could sell in a set period. Dealers have complained that this has overloaded the market and sent resale prices for many recent issues tumbling below the Mint's initial prices.

That appears to have discouraged many collectors and may account for some of the recent sales problems that the Mint has encountered. Sales of the 1989 congressional coins have been disappointing and, except for the silver Eagle, last year's sales of proof coins were well below 1988 levels.

In a speech in May to the Industry Council for Tangible Assets, Pope disclosed the new policy for proof sets and said she will announce the production numbers next month when the annual sales program is scheduled to begin. She also said that the Mint is worried about the large number of commemorative coins Congress seems to be intent on approving and said she would like to see lower production limits on these. "There is no point in saying we can mint up to 10 million coins when we know the most we are going to be able to sell is 1.5 or 2 million coins," she said.

THE 1990 Federal Duck Stamp goes on sale for $12.50 Saturday at 10 in the National Museum of American History's Carmichael Auditorium, 14th and Constitution NW. Call 208-4354 or 208-5508 for more information.

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