To mark its 40th anniversary, the United Nations this Wednesday will issue six stamps and three souvenir sheets that seem to look back almost nostalgically at the hopes that filled the world when the organization came into being.

On June 26, 1945, after three months of often difficult debate at a conference of 50 nations in San Francisco, the U.N. Charter was signed. Nazi Germany had surrendered during the conference and Japan would surrender almost two months later. The charter set forth goals of maintaining peace in a divided and troubled world and bettering the lot of humanity.

Two paintings by Andrew Wyeth, a world-famous American artist known for his meticulous rendering of quiet subjects, appear on the anniversary issues. The paintings, of rural scenes, convey a sense of peacefulness and tranquility that the world is still seeking. By coincidence, they were done in the early years after the Charter was adopted.

"The Corner," a night scene of a farmhouse and adjoining structures covered with snow, is filled with stillness and serenity and epitomizes the artist's gift for giving life to such inanimate objects as the weathered side of a building. It is on the three low values for New York, Geneva and Vienna. On the three high values is "Alvaro Raking Hay," a day scene of a motionless horse and wagon as a farmer engages in one of the most ancient and peaceful tasks known to man.

Another artist, Victor Vasarely, whose talents encompass painting, prints, sculpture, tapestry and architectural designs, has contributed one of his works for the cacheted covers and prints regularly put out by the World Federation of United Nations Associations to accompany new U.N. issues. Often called "Father of Op Art" and also a designer of stamps, he has contributed a symbolic painting of 16 different geometric shapes entitled "Plural."

In celebration of the U.N. anniversary, there will be two first-day cities in the United States on June 26, a practice that began in 1965 and has been continued every five years since. An agreement between the United Nations and the United States limits the use of United Nations stamps to its headquarters in New York. However, the United States has made five-year amendments to the agreement making San Francisco for one day part of U.N. headquarters, solely to allow first-day cancellations.

The six stamps in the anniversary issue consist of a 22-cent and a 45-cent for use at the New York headquarters and San Francisco, two stamps of 50 and 70 centimes in Swiss currency for use at the European headquarters of the organization in Geneva and two stamps of 6.50 and 8.50 schillings in Austrian currency for use at ancillary offices of the United Nations in Vienna. The U.S. equivalents of the Swiss values are 19 and 26 cents, of the Austrian values 29 and 39 cents, subject to exchange rate fluctuation.

The values of the souvenir sheets, each with a pair of stamps, are 67 cents for the New York two, 1.20 francs or 45 cents for the Geneva two and 15 shillings or 68 cents for the Vienna two.

Collectors wishing mint stamps, New York and San Francisco prepared first-day covers or prepared first-day covers with the Swiss or Austrian issues or additional information -- should send their orders to the U.N. Postal Adminstration, Box 5900, Grand Central Station, New York, N.Y. 10163-5900. There is a 50-cent handling charge for orders under $2.

Collectors sending in self-addressed first-day covers for the San Francisco or New York cancellation should send their orders to the above address. No orders should be sent directly to San Francisco. The New York headquarters will forward the orders to San Francisco for cancellation and return through the mail. The envelopes containing the covers must be marked "SAFDC-40th." The number of stamps or souvenir sheets to be affixed should be clearly marked in pencil at the upper right.

Payment must be by personal check, money order or certified check. Cash is not accepted. Orders for marginal inscriptions will be honored to the extent possible; orders for special combinations will be refused. The U.N. Postal Administration's new computer system is capable of handling mixed first-day cover orders for New York and San Francisco as well as mixed orders for U.S.-denomination first-covers and stamps.

For this anniversary issue, WFUNA's pie ce de re'sistance is a special "courier cover" that has all six stamps with first-day cancellations from San Francisco, New York, Geneva and Vienna, all dated June 26. The cover sells for $10.95. There is a postage charge of 50 cents for one to five covers, 20 cents for each additional five.

Orders for the courier cover and all other WFUNA material, or requests for added information, should be sent to WFUNA, Room DC1-1177, United Nations, New York, N.Y. 10017.

The organization's regular serviced cacheted first-day covers come in a variety of forms, with each of the six individual stamps, with both stamps of each issuing country, with the souvenir sheets and with plain or marginal inscription blocks.

A cacheted cover with the 22-cent stamp, canceled New York, is $1.10; canceled San Francisco, is $1.50; with the 45-cent stamp, canceled New York, $1.35, canceled San Francisco $1.75. Covers with the stamps in Swiss currency are $1.45 and $1.50; covers with Austrian denominations are $1.55 and $1.65.

In addition, WFUNA has a limited edition of 1,000 art prints 8 1/2-by-11 inches, hand-signed by the artist. They fit into standard albums. The prints come mint or with the 22-cent stamp, canceled first-day New York or San Francisco, the 50-centimes stamp or the 6.50-schilling stamp, canceled first-day Geneva or Vienna.