HK-485, SH 23-3, 1939 Golden Gate International Exposition Treasure Island or "Piece of Eight" Dollar.  Once considered perhaps the rarest 20th Century issue (other than others not then known to the authors) in the original Hibler & Kappen first edition, when perhaps a dozen were known of.  More have emerged since however certainly not common by any means, and a true rarity in Uncirculated grades.  Certified NGC AU-58, eight in the grade plus some at PCGS, 11 better at NGC (when last checked). These never really "circulated" as most were never sold, but acquired what wear might exist during storage.  Many have relatively poor Silver-plating that was easily bruised, and many tarnished badly - this is better than most.

NGC 6861479-006