Old and New London: Volume 1
There are four swords belonging to the City of London. The "Pearl" sword, presented by Queen Elizabeth when she opened the first Royal Exchange, in 1571, and so named from its being richly set with pearls. This sword is carried before the Lord Mayor on all occasions of rejoicing and festivity. The "Sword of State," borne before the Lord Mayor as an emblem of his authority. The "Black" sword, used on fast days, in Lent, and at the death of any of the royal family. And the fourth is that placed before the Lord Mayor's chair at the Central Criminal Court.
http://www.british-history.ac.uk/source.aspx?pubid=339
and on There was a reference as early as the 14th century in one source or another. The memento mori trends of objects and other references certainly way old too.
Cheers
GC