The promising envelope that carried everything has some very interesting bygone relics of U.S. Postage.
While I was checking this out, I came across the French commemorative of the same occasion (Union Postale Universelle), also issued 1963. A rather memorable event for the entire world. Thank you William for the great Mail Art (as usual). Always a grand affair when William visits the 4th Dimension. See you in the mail soon my old friend. Visit William's Mail Art site: Addressee.