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The buildup of heat has been a problem with underground electric cables and transformers from the beginning of centralized electrical distribution. This cover, un-identified or perhaps simply worn, tries to maximize ventilation while retaining the strength to hold up the weight of passing garbage trucks.
For an unidentified manhole cover this design is simple, efficient and attractive. It probably dates to the 1950's and 60's, when Con Edison seemed to be paying attention to good design in it's street hardware.
Most probably on a Con Edison cover
This stkyle turns up all around the city, this example is on Pineapple St. near Henry in Brooklyn Heights.