The system of merchant flags announced the approach of ships entering the Boston harbor. It was described in an 1890 book, by which time it had been superseded by telegraph communications:
Years ago, before the advent of the electric telegraph, a system of flag signals was in use, the merchants of Boston having a set of one hundred and twelve private signals, each one a different flag. When a ship arrived, the owner’s color was run up at Hull, repeated on an island in the harbor, again shown on Central Wharf, and finally at the old State House.
Condition: Generally very good, recently professionally cleaned and deacidifed, with some light remaining toning, discoloration, wear, cockling of paper. Vintage (possibly original) hardwood stained frame very good with the usual wear.
Tourists’ Guide to Down the Harbor, Hull and Nantasket… Boston: John F. Murphy, 1890. pp. 17-18. Online at Google Books: https://books.google.com/books?id=GtKC9Kur8dYC&pg=PA17 (31 August 2015).