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On the sign:
CITY OF WESTMINSTER JOHN STEPHEN 1934 - 2004 FOUNDER OF CARNABY STREET AS WORLD CENTRE FOR MENS FASHION IN THE 1960S
The plaque is located at the beginning of Carnaby Street and is dedicated to John Stephen nicknamed "King of Carnaby Street". The owner of a chain of clothing stores thanks to which Carnaby Street is one of the leading shopping streets in London.
In the next picture taken by the same photographer that day you can see the building and the sign on it Click for a larger image (On the left side of the picture you can see the entrance sign to the street, the same sign is at the end of the street, and is documented on the website Click for sign's details