The cover sits directly over the coal hole, a hexagonal cut in the bluestone sidewalk with no metal seating. Lines of small green and white glass "lenses" are interspersed with iron studs for protection. One wonders how much light actually seeped through.
Home delivery of coal was a growing business in the 1880's particularly is areas such as Brooklyn Heights.
This cover is on Willow St. in Brooklyn Heights between Clark and Pierrepont Streets. It is the only one with this design that I have encountered.