London Hallmarks
The London Assay Office is administered by the London Goldsmiths' Company. It has been in operation since the 1300s when a law of Edward I required all silver sold in England to be of at least the same quality as the silver currency (hence the term "sterling" for the silver standard). Originally the assayer would visit the workshops of the gold- and silversmiths to perform the assay. If items passed they were authenticated with the King's mark, a leopard's head. By 1478 it was deemed more practical for items to be brought to Goldsmiths Hall for assay and a permanent assay office was established there. This is the origin of the term "hallmark". In 1544 the leopard's head mark was adopted as the mark of the London Assay Office.
A letter mark coinciding with the date of assay was first introduced in London in 1477. Originally it signified the Assay Master responsible for testing and marking the silver. The date letters were changed on the day that the Goldsmith's Guild wardens were elected, originally St Dunstan's Day (19 May) until the Restoration (1660) and 29 May thereafter.
The Hallmarking Act of 1973 brought the remaining 4 British Assay Offices (London, Birmingham, Sheffield and Edinburgh) into line with each other with the date letter now changing (from 1975) on 1 January each year.