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On the sign:
CORPORATION OF LONDON
SITE OF KING WILLIAM STREET UNDERGROUND STATION FIRST CITY TERMINUS 1890 - 1900
The sign is in the place where the subway station stood - King William. The station, named after the street that runs above it, formed the northern terminus of the Northern line, and was the first railway station to be built at a deep level. The station only operated for about 10 years, and closed with the opening of the Moorgate extension.