The Hallmarking of silver and gold items has been in operation in the United Kingdom since 1327. Since that date it has been a punishable offence to offer for sale any article purporting to be made of a precious metal unless it bears the stamp (Hallmark) of one of the recognised Assay Offices.
A complete Hallmark comprises of up to six separate punch marks :-
1) The makers (or sponsor's) mark. This usually consists of the initials of the maker of the article or the person (sponsor) submitting the work for 'assay'. These initials are usually surrounded by a small shield or border.
2) Assay office mark.
This denotes the office which carried out the Assay. The most commonly seen marks are a Leopards Head (London)
3) & 4) The standard mark.This confirms that the article is in fact of the correct quality. For silver, the symbol is a Lion, one paw raised, facing left . For gold, various symbols have been used over the years, but that most commonly
seen is a Crown.
5) Date Letter. The date letter is the mark which is most useful to Horologists. Each of the Assay Offices uses a different letter each year. The yearly letter until recently varied from one office to another, so it is necessary to consult the lists for each particular office when dating an item.
The typeface, the case of the letter (upper or lower) and the style of the surrounding shield are all relevant, and should be compared carefully with the published lists.
6) Duty Mark.
This mark shows that duty was paid on this item. It appears on all articles made between 1st December 1784 and 30th April 1890, except on those articles not liable to compulsory Hallmarking and, of more interest to Horologists, on watch cases after 1798. The duty mark is the head of the sovereign reigning at that time.
More information can be found in one of the standard Hallmark books."Bradbury's Book of Hallmarks", (J.W.Northend Ltd.) and "Hallmarks & Date Letters" (NAG Press) are both readily available.
Useful link to the website of the British Hallmarking Council