The basic design is a bulls eye drain, a type that is occasionally seen to this day. It has somewhat amateurishly carved letters, note especially the A and R and the raised PT in DPT. However, it's the intricacy and delicacy of the casting that makes this cover stand out.
This cover is probably over one of the drain lines out of the reservoir carrying overflow water to nearby sewers. More specifically it is probably over a "silt well" a deep depression that would slow the overall flow of water and allow any debris to fall out. The silt would then be periodically cleaned out from above.
This beauty is just south of the East Drive between the pathway from Fifth Ave and the 85th St. transverse. It is probably unique today, however, about 50 ft or so north of this cover is an innocuous modern cover sitting in a 19th century captains wheel seat with 12 cogs , identical to the 12 around our 1862 standout.