From the website A London Inheritance:
“…And an FP plate on the left. According to a document on the Essex Fire Brigade web site, FP stands for Fire Plug. Apparently in the early days of the fire service, and when many underground water pipes were made out of wood, firemen would dig down to the water main and bore a small, circular hole in the pipe to obtain a supply of water to fight the fire.
When finished, they would put a wooden plug into the hole, and leave an FP plate on a nearby wall to alert future firefighters that a water main with a plug already existed.
When wooden pipes were replaced by cast iron pipes in the 19th century, workmen would often bore a small hole in the pipe and fit with a wooden plug when they saw an FP plate.
This would later be replaced with the Fire Hydrant method, which would be identified by a large H…”
Julia Upton 