SINCE 1956 the Postal Service has had something different to offer the thousands of people clamoring for commemorative stamps for their causes.

It's the commemorative postcard, an item that ranks somewhere between a commemorative and a special-event postmark.

Since almost anyone, from the Brady, Texas, Barbequed Goat Cookoff to the 100-year-old Choctaw, Mississippi Plaindealer, got a special postal cancellation this year, the offer of a commemorative postal card is something special.

Only about four are issued every year and the Service is committed to producing them with the same panache it gives stamps: the brighter and more colorful the better.

Thanks to the multi-color presses at the Government Printing Office, and the bright white stock on which they're printed, the new cards are a far cry from the dull penny postcards of the past. But, then, at 14 cents each, a little color may not be too much to ask.

This year's postcard topics have ranged from the 200th anniversary of the Constitutional Convention to the 150th anniversary of the self-scouring steel plow, the sodbuster that made possible large-scale farming on the plains of the Midwest.

The latest card, created by graphic designer Lou Nolan of McLean, is a tribute to President Reagan's "Take Pride in America" campaign. The card, to be issued September 22 at the National Elk Refuge in Jackson, Wyoming, stresses the Interior Department's efforts to care for the nation's public lands and parks.

Nolan, who has designed three stamps for the Postal Service, said his card represents a typical -- but not actual -- Western mountain scene. "It's quite an honor to be asked to do one," said Nolan, who also created this year's CPA commemorative stamp.

The "Take Pride" card will debut in a first-day ceremony at which Interior Secretary Donald P. Hodel will be a featured speaker.

Another postal card, a variety of the 14-cent U.S. flag card issued in Baltimore June 14, will debut in Washington September 1 without ceremony. This card is a 28-cent "double-reply" card that has one 14-cent card for the sender's message and an attached reply card.

Washington collectors can share in the pomp and circumstance of a formal first-day ceremony on Friday, August 28, however, when the previously announced booklet of 22-cent commemoratives celebrating the bicenntenial of the Constitution goes on sale. That event will be held in the Rotunda of the National Archives on Constitution Avenue NW at 9 a.m. Postmaster General Preston R. Tisch and Frank G. Burke, the acting archivist of the U.S., will be among the speakers.

The five 22-cent stamps being issued at the ceremony carry the Preamble to the Constitution. The stamps, which feature a golden eagle and the words of the preamble, are being issued in booklets of 20 stamps, with five stamps to a "pane," or sheet.

Collectors who want first-day cancellations of the postal cards will have to act by October 1. Addressed "Take Pride" cards should be mailed to Customer-Provided Cards, Postmaster, Jackson WY 83001-9991. Up to 50 cards may be purchased from Take Pride in America Postal Cards, Postmaster, Jackson WY 83001-9992 for 14 cents each.

The double-reply flag postcards may be ordered for 28 cents each from U.S. Flag Reply Postal Card, Postmaster, Washington DC 20066-9992. For each card, collectors are asked to provide peelable return address labels.

The Postal Service will affix only the full pane of five Constitution stamps on first-day envelopes at a price of $1.10 each. The address for that service is Constitution Booklet Stamps, Postmaster, Washington DC 20066-9992. Customers may send envelopes with the stamps attached to Customer- affixed Envelopes, Constitution Booklet Stamps, Postmaster, Washington DC 20066-9991. The deadline's September 27.

Bill McAllister is a member of The Post's national staff.