China shows list of Treaty Ports open to foreign trade 1883 Lett's SDUK mapChina. (Asia). Issued 1883, London by Letts, Son & Co. Attractive and fascinating 19th century color lithographed map. Below the title is a detailed key which includes listing the Treaty ports then open to foreign trade. A late re issue of the SDUK plates from the 1830's & 40's, here transferred from steel to a lithographic process of printing, with amendments made to the maps during the process. Hence, Lett's firm did not merely re issue the SDUK