AU55 NGC, KM528, Sch-407. Obv. Tu Duc Thong Bao. Rev. Inscription, Tam Thao above three Longevities. Iridescent, minimally marked, surfaces, with light marks and a somewhat uneven reverse strike. A very rare issue and only the second example we have seen. 

Heritage Auctions. 2016 December 8 - 9 HKINF World Coins & Ancient Coins Signature Auction - Hong Kong #3050