Honeycombs break out
The wheels of hexagons, the honeycomb pattern, moved beyond the borders of Brooklyn in the 20th century, and became synonomous with Sewers. This one has only a vestigial, and superfluous S.
In New York at least, the design became standardized. Nineteen solid hexagons in three circular rings, with single letters imprinted in individual hexagons.
This was probably cast by the consolidated Department of Public Works before the Borough Presidents started branding their borough's covers.
This cover is on Broadway about 119th St. in Manhattan, but because the S hexagon is easily worn down, many seeming blank covers throughout the city may be of this type.