1899, New Zealand, Great Barrier Island, "PIGEON OVERPRINT" used on (reinforced) flimsy, neatly oblit. by violet rubber circ. "GREAT BARRIER / THE ORIGINAL / PIGEONGRAM SERVICE", signed and certificate RPS. The most expensive value of Pigeon Post stamps - on entire a desirable rarity, only very few are known! (ACS $10000 for used single - no prices are given for entires) (Image)