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The plaque is in this passageway close to the entrance photographed.  The sculpture panel is further in, attached to the wall on the left, the boundary wall with St Mary Woolnoth.

Before the Great Fire of London Sir Robert Vyner had a large house here giving on to Abchurch Lane and Lombard Street, and it is known that, after the fire, the GPO moved here from a site in Bishopsgate Street, either rebuilding or re-purposing Vyner's house. This c.1894 map shows the “Site of old General Post Office” and also a current (1894) “Post Office” stretching between Lombard Street and King William Street. 

In 1829 the GPO departed for St-Martin's-le-Grand and left behind a branch PO which remained until the 1990s. Perhaps what happened in 1951 is that the last remaining fragment of the old GPO buildings was demolished and replaced.

Alamy have an illustration of "The old Post Office in Lombard Street about 1800" showing it tight up against St Mary Woolnoth. Drawings of this type were often created when it was known that the old building was about to be demolished.

Getty have an image "late 19th early 20th century" which looks more contemporaneous with the sculpture but it is very plain with no decorative architectural features visible. It probably shows the open space that can be seen on the map.