This PRR (Pennsylvania Rail Road button is a two-piece, convex, nickel-plated button with brass back and loop. Button dates from pre 1920.  Unique back mark reads, "AMERICAN RY. SUPPLY CO. 132 CHARLES ST".  Button measures 15/16" in diameter. 

'In 1846 PRR took over the state owned system of canals and railroads known as the Main Line of Public Works.  From this, it grew rapidly to a state-wide system reaching New York, Chicago and west to St Louis.  For many years the different operating lines control by the PRR retained their individuality and their own buttons.' BBB p. 763.