Shanghai had developed a postal service as early as the Ming Dynasty, but during the treaty port era foreign postal services were organised through their respective consulates. For example, the United States Post Office Department maintained a United States Postal Agency at the Shanghai consulate through which Americans could use the US Post Office to send mail to and from the US mainland and US territories. Starting in 1919 the 16 current regular US stamps were overprinted for use in Shanghai with the city's name, "China", and amounts double their printed face values. In 1922 texts for two of the overprints were changed, thereby completing the Scott catalogue set of K1-18, "Offices in China".