Nowhere else in the world are consumers more highly protected than in the UK. The UK is one of only a few countries in the world that have compulsory statutory hallmarking. This means that every item sold as precious metal, ie gold, silver, platinum or palladium must have been tested and hallmarked by an independent third-party Assay Office to guarantee that the precious metal is of the fineness stated.
It shall be an offence for any person knowingly or any dealer to supply or offer to supply any article bearing any mark of the character of a hallmark and if the article is in the possession of an assay office may be cancelled, obliterated or defaced.
The Antique Plate Committee (APC) of the London Goldsmiths' Company, created in 1939, is the internationally renowned body for adjudicating spurious silver articles. It advises the Assay Office on the authenticity of an article or whether there has been an illegal alteration or an addition to the piece, whether it could be an electrotyping, or if the hallmarks have been transposed or cast.
If an article submitted for examination bears forged, counterfeit or transposed marks, they are cancelled. Any such mark which, in the opinion of the Committee, was struck upon the article or transposed into it before 1950 is also cancelled.
When the marks have been cancelled the article is returned to the owner marked LAO and case number or the owner has the option of submitting the piece for hallmarking as a new article, and being charged accordingly.

Example of plate with cancelled 18th century hallmarks and 2018 LAO new hallmarks
Example of pitcher with cancelled 18th century hallmarks (LAO 8908)

The case of LAO 9191
In England duty has been paid as a tax on silverware since 18th Century. The tax was paid at the time of assay, and the amount due related to the weight of the article. From 1784 until 1890 a mark with the head in profile of the Sovereign in escutcheon was punched on articles to attest that the excise duty had been paid.
"Duty dodger" is the definition of unscrupulous silversmiths that used several methods to avoid paying the tax, eg inserting into a new article an existing set of hallmarks removed from an old piece.
An example is the case number LAO 9191, interesting a teapot having the bottom with hallmark transposed from a George III piece (possibly a family property refashioned in Victorian era).
The teapot bears on the bottom cancelled Georgian hallmarks (date "t", lion passant, leopard's head, duty mark and maker's mark HC) with the addition of "LAO" mark (London Assay Office) and 9191 (case number). No hallmark is present on the lid and on the finial.
The mark HC belongs to Henry Chawner (1764-1851) who registered these marks in 1786 and 1787 (from 1796 until 1808 Chawner used a different mark in partnership with John Emes).

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This is a page of 'The What is? Silver Dictionary' of A Small Collection of Antique Silver and Objects of vertu, a 1500 pages richly illustrated website offering all you need to know about antique silver, sterling silver, silverplate, Sheffield plate, electroplate silver, silverware, flatware, tea services and tea complements, marks and hallmarks, articles, books, auction catalogs, famous silversmiths (Tiffany, Gorham, Jensen, Elkington), history, oddities ...