Who's In Charge Here
In the early 20th century the Bureau of Sewers was part of the Department of Public Works. Is there anyone now who remembers the wrangling and plots that went into placing the Department's name on this manhole cover rather than the Bureau's. Or was it the other way around? Sic Transit Office politics.
This is the same design as the covers labeled Bureau of Sewers Borough of Manhattan, circa 1900 - 1920.
The inscription does not explicitly say SEWER, but the design incorporates the wheel and spoke motif (here with 10 spokes) that had been taken up by earlier sewer cover designers. Also the large vents are inviting storm runoff to enter.
This example comes from the playground at W 143rd St. and Riverside Drive. There are quite a few examples of this type of cover still extant in Manhattan, but often the lettering is obscured -- so the balance between "Bureau's" and "Dept's" is not clear.