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On the sign:
Single-fluke anchor, about 1820
A pattern still used for permanent moorings, frequently in association with ship mooring buoys. It has no upward pointing fluke for a vessel to sit on with a falling tide.
This one was found in the mud at Deptford, less than a mile from here. It may have been used in hauling vessels off the slipway at the former Royal Navy dockyard.
The large anchor ring is typical of the period; it was usually bound with rope or canvas to prevent chafing of the hemp cable.