Brooklyn Water Works
The Brooklyn Water Works was incorporated in 1857 as a private company, which was largely owned by the City of Brooklyn. Brooklyn at that time was acutely aware of the progress that New York City was making in supplying water through the Croton Aqueduct, and wanted to insure an adequate supply for its own growth. The Ridgewood Reservoir was built which collected water from wells and ponds in Nassau County (think Conduit Ave) and then distributed the water to the City. The BWW was succeeded by the Board of Water Supply and Sewerage Commissioners in the 1860's and then by the Department of City Works in 1872. This BWW cover and a few of its nearby duplicates appears then to be --The Oldest Manhole Covers in Brooklyn!
One hint that this is a real oldie is that the rim of the cover overlapped the seat. -- a design that raised the lid above the surrounding fill, and which was largely replaced early on with covers sitting on a recessed edge with of the surrounding rim. Rim and cover were then level with the surrounding fill, and much less likely to be tripped on.
The BWW was responsible for both water supply and sewer service. This cover is at a veritable water supply hot spot, being also the site of the end of the Catskill Aqueduct completed much later.
This cover is at an entrance to Fort Greene Park at Myrtle Ave and Washington Park. Two other similar covers are nearby at Ashland Place and Willoughby St. There may be more!