Catalogs:

• Novak: #109b
  dao wrongly read as thong
• Thierry: #1120
  Brass. D = 23 mm. W = 1.61 g
  Imitation of the Chinese cash coin Zhi-dao yuan-bao (995-996 AD), Emperor Tai Zong, Northern Song Dynasty.
  Special calligraphy of the dao.
  XVI-XVII^th Century.
• YNLSHB: p.472, coin #7.
  Uncertain origin.

References:

• Novak, John A. A working Aid for collectors of Annamese coins. Longview, Wash., USA, 1967.
• Thierry, François, Catalogue des monnaies vietnamiennes, Bibliothèque Nationale de France, Paris 1987. In French.
• Tang Guo Yen, Chang Shi Chuan Yuenan lishi huobi (in Vietnamese Lich suu dong tien Vietnam - The Vietnamese historical currency), 1993, published by Yunnan and Guangxi Numismatic Society, in Chinese.