B for B LIne?
This honeycomb cover may have a date stamped on it "1900" but certainly comes from that era, right after consolidation of Brooklyn and New York City. The meaning of the "B" is uncertain, There is a string of similar covers in Windsor Terrace with a "C" in the center, and Diana Stuart in her book "Designs Underfoot" suggests these may be line designations.
This is an early example of the ring of six vent holes around the middle ring of cells, that is widely seen in later examples.
This cover with the letter B was seen on Stratford Rd between Church and Albemarle Rd in Brooklyn. A few others with a different style B can be seen in Bensonhurst.