Thursday might well have been observed as Odd-Coin Day, if numismatic circles were alert to the changes Treasury has historically sought to put into effect to meet great needs in circulating currency. The lessons learned in the 19th century are, very possibly, going to have to be relearned.

On March 3, 1851, a 3-cent coin was authorized to reduce the demand for the moderately large copper cents. The new denomination appeared first in silver (1851-1873), with a star on the obverse, and the Roman numeral "III" within a large "C" on the reverse. Peak production years were 1852 (18 2/3 million) and 1853 (11 1/2 million).

Thereafter the million-mark was attained only in 1856, 1857 and 1858. After that (1859-62) minting dropped below the 500,000 mark, and only proofs were struck from 1863 to 1873.

Undaunted, Treasury turned to nickel in 1865, and struck a new design with Liberty on the obverse and "III" on the reverse. Here, too, the initial striking of 3-cent coins in 1865 was exceptional.

Totals produced dropped to 4 3/4 million in 1866, and declined annually to 1 1/3 million in 1869. Thereafter, the million mark was attained only in 1873 and 1881. The odd-denomination coin disappeared after 1889.

Within that period Treasury came up with a 20-cent coin, to reduce the demand of dimes. The 20-cent piece was minted in Philadelphia, San Francisco and Carson City to the extent of roughly 1 1/4 million, and literally died. The experiment of March 3, 1875, proved a dud.

On March 3, 1849 a gold dollar was authorized. This coin was very tiny, and wasn't well received for the first three years. Treasury did push out about 3 2/3 million in 1851, somewhat over 2 million in the following year and about 4 million in 1854 - but despite a size increase in that year, the distribution (and production) dropped very sharply. The million-mark was exceeded thereafter only in 1856 and 1862, and the remaining years (1863-1889) saw what may best be described as token striking (198,800 to 400,000).

Visitors to the nation's capitol have two opportunities to view inaugural medal displays. On such exhibit is in the Smithsonian's Museum of History and Technology in an area adjoining the Numismatic Hall.

The other is in the Smithsonian's National Portrait Gallery, and was organized by Neil MacNeil, a leading authority on inaugural medals. According to Marvin Sadik, director of the Gallery, its showing represents the first complete study in this area.

A comprehensive catalog has been produced for the National Portrait Gallery by McNeill (Clarkson N. Potter, Inc., publisher). The 160-page guide, with 118 black-and-white illustrations and four pages in color, is now available at $12.95 for a hardcover version and $8.95 in softcover (plus $1 for mailing and handling) from the National Portrait Gallery), Smithsonian Institution, Washington, D.C. 20560.

Jamaica has produced a new $10 sterling silver coin, portraying Admiral George Rodney - who saved the island from a French invasion in 1782 - on the obverse. His 90-gun flagship, "H.M.S. Formidable," appears with Rodney's likeness.

The new coin is included in the 1977-dated, nine-coin Jamaican proof set, and is also available as an individual proof, through Paramount International Coin Corporation, Englewood, Ohio.

Cook Islands authorized specially minted limited collectors' editions of its coinage commemorating the silver jubilee of the accession to the silver jubilee of the accession to the throne of Queen Elizabeth II. The $25 silver and $100 gold coins have been offered in proof and uncirculated versions through the Numismatic Bureau, Cook Island Treasury, Post Office Box 11, Raratonga, Cook Islands.

Liberian 25-cent, 50-cent and $1 coins in a 1976-dated proof set will bear new designs. The 25-cent item will show a Liberian woman carrying a basket of harvested rice, and is linked with FAO's "Grow more food" program.

Liberia's great seal and national motto will be highlighted on the new 50-cent piece, while the $1 coin will stress a relief map of the West African republic. Paramount International has been named distributor of the proof sets.

The American Bicentennial Commemorative Society has released its 23rd medal, honoring Alexander Hamilton, first United States Secretary of the Treasury. STAMPS

Two issuance months for scheduled United States postal items, and three revised appearances of previously announced commemoratives, have been announced by the Postal Service.

The Talking Pictures golden jubilee commemorative will appear this October, and the Galveston Court House postal card will be released during July.

Colorado's Statehood Centennial commemorative has been rescheduled from March to May - possibly in conjunction with the popular Rompex show in Denver. The unit of four Indian Pottery stamps, announced for March also, will now appear in April, and the four Butterfly stamps will be issued in June rather than May.

The Bureau of Engraving souvenir card, released Friday at the Milwaukee Philatelic Society Milcopex '77 show, is now being mailed to those who ordered their copies by mail. The dominant illustration is an enlargement of the upper segment of the 4-cent Arctic Exploration commemorative of 1959, based on the original working model.

Copies of the Polar Exploration souvenir card may be ordered at $1.50 apiece from "Milcopex '77, Bureau of Engraving and Printing, Washington, DC 20228." There is a limit of five cards per order through April 2.

For the following 60 days (through June 1) unlimited quantities may be ordered, unless stocks are depleted earlier. Payment should be by check or money order.

Two major first-day sales will get the 19th annual Interpex show off to a good start Friday at the Americana Hotel, Seventh Avenue and 52nd Street, in Manhattan. The United States Postal Service will release a new $1 booklet pane that day, and the United Nations Postal Administration will issue its three World Intellectual Property Organization stamps and a souvenir card.

Another souvenir card, recognizing the 50th anniversary of Charles A. Lindbergh's solo transatlantic flight, will be offered by the sponsoring American Stamp Dealer's Association.

Show hours will be from 10 a.m. to 9 p.m. Friday; 10 a.m. to 7 p.m. Saturday, and 11 a.m. to 6 p.m. next Sunday. There will be a $2.50 admission charge for adults ($1.25 for children).

Most major stamp societies will have booths and special programs at the show. The Aerophilatelic Federation of the Americas will be a major participant, and the Americans Academy of Philately will offer a special symposium on judging Saturday at 2 p.m.

Members of The Collectors Club will participate in another in a series of annual competitions at Interpex, providing the window-dressing for the large number of dealer booths and postal administration sales areas.

USPS' contribution to this year's Interpex show is a $1 booklet designed for use in vending machines. Each booklet contains a single pane of eight stamps - seven of the new-design 13-cent "Flag Over the Capitol" adhesive, and a single 9-cent "Dome of the Capitol" variant of the previously issued "Americana" design.

The single 9-cent stamp will represent a distinct variety, since it will be printed in green on white stock. In its present sheet form, printing was in green on gray paper.

Postal Service is offering first-day cover collectors a plain 6 1/2-by-8 1/2-inch envelope bearing the full booklet pane with the first day cancellation at face value. Orders should be sent to "Booklet Pane Covers, Philatelic Sales Division, Washington DC 20265." There will be no service charge, and orders may be postmarked as late as March 26, but it is requested that no cash be sent with orders.

Collectors may, of course, prepare their own first day covers. Those who cannot purchase the new booklets locally may order them from the Philatelic Sales Division ($1 Booklets), Washington, DC 20265. There is a 50-cent charge per mail order.

These orders will be expedited, so that individuals may affix their own panes to their own covers, and mail them to the Postmaster, New York, NY 10001, no later than March 26.