WHEN VISITORS to last fall's World Stamp Expo stopped by the Postal Service's flashy video wall, they not only got a fast-paced look at the country's history as portrayed on its stamps, but also a sneak preview of the designs for several upcoming commemoratives.

Last week, seven months after the show closed, the Postal Service formally announced first-day plans for the last five of the previewed stamps, a set that now seems likely to be a harbinger of commemoratives in the future.

The stamps, featuring the headdresses worn by leaders of five Plains Indian tribes, are the first in the Postal Service's long-running American folk art series to be issued in booklet form. That's a format that many traditional collectors detest, preferring their commemoratives in sheet formats.

But, under the terms of the new stamps printing contract the Postal Service recently signed with the Bureau of Engraving and Printing, it is a format that will be increasingly used for commemoratives. Postals executives say that's what their customers want and the new contract calls for sheet stamps to compose only 15 percent of the bureau's fiscal 1991 stamp production; sheet commemoratives will compose only 20 percent of those. Booklets, including commemoratives, will compose 30 percent of the total production.

The headdress stamps also reflect the Postal Service's desire to issue more stamps with Western themes, an interest that seems likely to grow with the appointment of Wyoming broadcaster Jack Rosenthal as head of the Citizens' Stamp Advisory Committee.

Rosenthal was instrumental in getting Wild West showman William F. "Buffalo Bill" Cody on the 15-cent stamp, released at the Buffalo Bill Historical Center in Cody, Wyo. The center owns four of the headdresses pictured on the new stamps and will be the site of their release on Aug. 17

The 25-cent stamps display headdresses of five tribes -- the Assinibonie, Cheyenne, Commanche, Flathead and Shoshone. All of the tribes had bonnets made of hawk and eagle feathers, beads, strips of animal fur and horsehair, similar to the headdresses Hollywood has fooled the world into believing all American Indians wore.

Lunda Hoyle Gill of Riverside, Calif., designed the stamps from the Cody headdresses and a privately owned Comanche bonnet.

The stamps have been revised slightly since their designs were flashed on the Stamp Expo video wall. But the revisions are minor, according to postal spokesman Michael O'Hara.

The fire-red feathers on the Comanche headdress and the light blue ones on the Flathead appear to have needed little help, but the other headdresses are dominated by black and white feathers and appear less dramatic.

The stamps are being produced by the Bureau of Engraving in $5 booklets of 20 stamps each. Four of the bureau's best known stamp engravers -- Gary Slaght, Thomas Hipschen, Kenneth Kipperman and Gary Chaconas -- were called on to prepare the stamps along with modeler Peter Cocci, also a well-known stamp designer.

Since the stamps appear side by side in strips of five, as did this summer's stamps honoring Olympic athletes, they will offer another headache for first-day collectors. Those who wish to get a single strip on one envelope will have to prepare their own, making certain that it is large enough to carry the 7.8-inch long strip. The Postal Service will affix only single stamps on envelopes, so collectors will have to supply at least five envelopes unless they choose to purchase the stamps at their local Post Office after Aug. 17.

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