The 10-cent Preserve the Environment commemorative stamp was first placed on sale at Spokane, Washington, on April 18, 1974. The stamp commemorates Expo '74, which was held in Spokane, Washington, at the time the smallest city to have hosted a world's fair. The theme for the fair was preserving the environment.
The stamp was designed by Peter Max and was issued in sheets of forty, with an initial printing of 140 million.
Postal Bulletin (March 7, 1974).
Center for the Study of the Pacific Northwest, University of Washington. "History of Washington State & the Pacific Northwest." http://www.washington.edu/uwired/outreach/cspn/Website/Course%20Index/Lessons/26/26.html.