The 5-cent Mary Cassatt commemorative postage stamp, the sixth in the annually-issued American Painting Series, was first placed on sale in Washington, DC, on November 17, 1966. One of Cassatt's paintings — "The Boating Party" — inspired it. The original hangs in the National Gallery of Art in Washington, DC.
Six inks were applied in two passes through the Giori press, and these were blended and overlaid to create many additional tones. Designed by Robert J. Jones, the stamp was issued in panes of fifty and authorized for an initial printing of 120 million.
Postal Bulletin (October 13, 1966).