The Neenah Foundry is one of the largest man-hole cover manufacturers in the US, but, like the BWS, has only a limited presence on the streets of New York. It is also one of the few foundries to have a video about the design history of the firms manhole covers! Neenah Video . This cover is their Cope B "Waffle" design, made since 1934.
This cover probably allows access to valves along the Third Tunnel adjusting water levels to lower Manhattan and Queens.
This cover is on the 79th St. Transverse of Central Park, just east of the East Drive bridge and is unique as far as I can tell.