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On the sign:
Ordnance Survey Bench Mark The small plate (marked G 1692) is an Ordnance Survey benchmark. Similar marks appear on walls and buildings across the country. The height of each mark above sea level has been measured and recorded. Dating from the 1940s, this particular mark is a replacement for an older one that once existed nearby.
Public Standards of Length These British Imperial Standards were first mounted outside the Observatory main gates in January 1859 to enable the public to check measures of length.
The stated length is the distance between the inner faces of the two D-shaped studs.