Two-piece low-convex brass uniform button, coat size: 22 mm. Front: rope border outside a plain border, lined field, the letters "O.C.S. B.Co." According to Van Court, "This design was worn on famous Fall River Line boats", page 244 volume one. And, he says: the company was formed in 1877 by the Old Colony RR. Back mark: " Waterbury Button Co. Waterbury Con.". dm in between two lines. This back mark used 1866-1900, according to Tice page 84
The Fall River Line was a combination steamboat and railroad connection between New York City and Boston that operated between 1847 and 1937. It consisted of a railroad journey between Boston and Fall River, Massachusetts, where passengers would then board steamboats for the journey through Narragansett Bay and Long Island Sound to the line's own Hudson River dock in Manhattan. For many years, it was the preferred route to take for travel between the two major cities. The line was extremely popular, and its steamboats were some of the most advanced and luxurious of their day.
I hope that's right nice button!