Essay: any design, or part of a design, essayed to or produced by a government (or established mail carrier) for a stamp and differing in design in any particular from an officially issued stamp. Any print, not a stamp sold by the Government, printed from an officially approved design die or plate at any time during the existence of the die or plate, is a proof, no matter in what color, or material upon which it may be printed for experimental purposes. All essays and proofs are printed only by the proof printer, or proofer, on a hand press.