I just received my first Sierra Club label today on a mailing soliciting membership and my signature on a petition to the Speaker of the House and Senate Majority leader.
For those who may not be aware, the Sierra Club was founded in 1892 by the noted naturalist John Muir, who was honored with this 5c commemorative on April 29, 1964 (Scott 1245).

Muir was apparently a friend of President Teddy Roosevelt and played a role in advocating for establishment of the National Park System.
The words on the label are taken from the Club's motto: "Explore, enjoy and protect the planet."
Steve