A Croton Water Distribution Line Cover
The Croton Aqueduct Department operated from April 11, 1849 through 1869. In 1870 it was subsumed under the newly formed Department of Public Works (headed by one William Marcy Tweed.) The CAD inscription seems to be a unique survivor although it is along a line of similarly sized covers on Broadway emblazoned with CROTON WATER surrounding a DPW medallion.
This design, a bold emblem CAD on a field of angled cross hatches, is quite rare, with only a single example of a close relative - a cover emblazoned with DPW on the same cross hatching, seen in Central Park. The similarity in design suggest that both covers were made close to the transition time between the CAD and DPW (circa 1870).
Croton Aqueduct Department (New York City) These were found along water distribution lines coming from the Manhattan reservoirs, as opposed to the slightly larger Croton Aqueduct Department styles seen along incoming lines.
The only example of this CAD style I have seen is this one on Broadway at 141st St.