This classic cover dates from the early 1920's or so. Flockhart Foundry was established in 1918 (succeeding Maher and Flockhart) and the era of ornate monogram-like lettering was at its peak. The BS refers of course to the Manhattan Bureau of Sewers
Seven rings of raised iron nubs are practical,and give an almost regal air to this handsome cover.
This item was spotted in Stuyvesant Square, Manhattan. Other examples are spread over the entire borough, although not all of them have the foundry inscription seen here on the sewer-head rim.