HPFS in Brooklyn
In 1904 the rage for High Pressure Fire Service water lines was as strong in Brooklyn as in Manhattan, perhaps even more so given the lower overall water pressure. Landowners liked them because it promised to lower their fire insurance costs, politians liked them because there were so many important decisions as to exactly which streets would get the service, not to mention all the contracts that would need be let.
This small survivor (it's about 12 inches across) was probably a valve cover for a connection to an HPFS hydrant - similar to the ubiquitous WSNY boxes near normal pressure hydrants throughout the city today.
This cover is in the middle of Hudson St. between Fulton and Dekalb, in Fort Greene, Brooklyn. When the HPFS hydrants were finally removed in the 1980's the associated manhole covers were to be removed as well, but a few survive.