[6][7] The Tự Đức Bảo Sao or Đồng Sao (銅鈔, billets of copper) were introduced by the Ministry of Revenue (戸部, Hộ Bộ) in the year Tự Đức 14 (1961) for large transactions and taxes on behalf of stores of the government of Đại Nam, the introduction of the Tự Đức Bảo Sao marked the redefinition of the tiền or mạch denominations and the quàn (strings of cash coins) where the quàn was made equal to 10 mạch and the mạch was made the equivalent of 60 zinc cash coins, under these exchange rates 1 quàn was worth a string of 600 zinc cash coins.

[8][9] The Tự Đức Bảo Sao was generally well received by the population of Đại Nam despite the fact that their circulation was reduced due to their high purchasing power relative to their intrinsic value until their weight was decreased, which was done by the government to conform to the new official exchange between brass and zinc cash coins.

[8] From September 1870 the Tự Đức Bảo Sao series of cash coins has had their weight decreased, but kept the same nominal values and denominations.

[8] French author Albert Schroeder referenced the correct reading being mentioned in a decree issued in 1861 by the government of the Nguyễn dynasty.

[8] The Tự Đức Bảo Sao with nominal values higher than 60 văn have reverse inscriptions that are read in the typical manner (top-bottom-right-left), for example Chuẩn đang nhị mạch (準當二陌) rather than Chuẩn nhị đang mạch (準二當陌) if read using the above method.