The U.S. Postal Service will issue four commemorative stamps in the Constitution Series honoring the three branches of government - executive, legislative and judicial - established by the U.S. Constitution 200 years ago. The four issues will be based on a similar design.

The first issues will honor the legislative branch, with a pair of 25-cent stamps honoring the U.S. Senate and the House of Representatives. The stamps will be issued in conjunction with anniversary events celebrating the first quorums achieved by both Houses of Congress.Several common elements are used in the designs of the Congressional stamps. Sculptures associated with each legislative branch appear in beige set against a dark blue background. The name of the institution is seen in red lettering against a stripe of beige above the vignette. The design is framed by a stripe of red at the top and bottom, with "Bicentennial" appearing in white letters in the top stripe.

The House of Representatives stamp features the 1819 marblework clock of Carlo Franzoni. This clock is now on display above the north entrance to Statuary Hall in the Capitol. The muse Clio, representing History, stands in the winged Chariot of Time, recording events as they occur. In the upper right, "USA" and "25" appear in two lines of white against a blue background.

The Senate stamp depicts the carved gilt eagle and shield located above the vice president's chair in the Old Senate Chamber. "USA" and "25" appear in two lines of white lettering in the bottom right of the blue background.

First-day cancellations of both stamps are available separately in either of two ways.

The Postal Service urges collectors to buy the stamps at their local post office and affix them to their own self-addressed envelopes. Such orders receive preferential service and require no remittance.

Send self-addressed envelopes bearing the House of Representatives stamp to Customer-Affixed Envelopes, U.S. House of Representatives Stamp, Postmaster, Washington, DC 20066-9991. Envelopes with the Senate stamp should be sent to Customer-Affixed Envelopes, U.S. Senate Stamp, Postmaster, Washington, DC 20066-9991.

If you prefer to have the Postal Service affix the House of Representatives stamp, send a personal check or money order for 25 cents per stamp to U.S. House of Representatives Stamp, Postmaster, Washington, DC 20066-9992. The Senate stamp can be acquired in the same manner, except that requests should be sent instead to Senate Stamp, Postmaster, Washington, DC 20066-9992.

The deadline in all cases is May 4.

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New collectors may be interested in an offering from the U.S. Stamp Collectors Society, which is making available a special collection of 300 all-different U.S. stamps issued during the last 100 years. Included is one of the first commemoratives issued for the Columbia Exposition of 1892. The cost is $10.95. Also available is a 16-page, 9- by 12-inch album to accommodate the stamps, for $16.95. Stamps and album can be ordered together for $25.95. Write to U.S. Stamp Collectors Society, P.O. Box 854, Van Nuys, CA 91408.

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Looking ahead to 1990, the Scott Publishing Co. has announced it will change its stamp values to reflect actual dealer selling prices rather than a flat catalog price.

This major change will be seen in the new Volume I edition, which covers the United States and British Commonwealth. It is scheduled to be issued in June.

This departure in pricing by Scott is a welcome arrangement for collectors everywhere. The new 1990 values will be based on a comprehensive study of hundreds of dealer retail-price lists, public auction sales and advertised prices.

In other words, the retail values listed in the catalog will be what collectors could expect to pay for a stamp of fine grade in the marketplace.