The colony of South Australia began issuing postage stamps in 1855, and continued until the first stamps of the Australian Commonwealth were issued in 1913. Many of these stamps were intended for use on official mail by government departments, and were overprinted with various initials to prevent unofficial usage by staff and to identify the department concerned. The overprints were in use from 1 April 1868 until 1874, when they were replaced by the letters "O.S." which was then used on official mail of South Australia. Red and black were the most common colours for overprints, but blue was also used.