In the 19th century, when New York and Brooklyn were separate cities, New York was not shy at all about putting dates on its manhole covers, and there are dozens of examples from the 1860's and 70's still seen on sidewalks today. Brooklyn, on the other hand was a bit more coy, and there are only a few covers with just the traces of the year they were forged to be found (see below). The big bold 65 on this beauty stands out. The design and locations could be from the 1800's, (Brooklyn began building citywide sewers in 1857) but the lack of an 18 in front of the 65 leaves the question of the actual date still unknown.
The wheel and spoke design became well established in New York from it's use by the Croton Aqueduct Department. The circle of balls or vent holes was also a common motif, but seems concentrated in Brooklyn more than the other boroughs, seemingly indicating a pre-consolidation preference.
Probably a sewer cap.
This example is found in the circle at Prospect Park West and 15th Street in Park Slope. Others with the same basic design are found throughout the borough. There may well be other covers with big numbers about, keep looking!