Each of the three 1973-1974 Rural America stamps celebrated a different theme associated with life in rural America. The different rates, eight cents or ten cents, deserve mention. The first stamp issued, the 8-cent Angus and Longhorn Cattle Rural America, was issued in October 1973, when the first-class domestic letter rate was eight cents. However, the Postal Service released the other two stamps ten months later, in August 1974. On March 2, 1974, the first-class domestic letter rate had increased to ten cents, resulting in two different valuations for the Rural America stamps.
Kloetzel, James E. 2009 Specialized Catalogue of United States Stamps and Covers. 87th ed. Sidney, Ohio: Scott Publishing Co., 2008.