The collector John W. Simpson was probably John Woodruff Simpson, a New York based lawyer. He was called to the US bar in 1873 and practised with the firm of Alexander & Green (later Simpson, Thacher and Barnum) from 1884. He was an admirer of JW's works and is likely to have met him in 1902 (see #01376).
Who Was Who in America, vol. 2, 1897-1941, Chicago, 1943.