A William Swan is listed in Munk's roll, Lives of the Fellows of the Royal College of Physicians, vol 1, p315: "William Swan, A.B. - A bachelor of arts of Cambridge, educated at Emmanuel College, and then practising at Chadwell, Essex, was admitted an Extra-Licentiate of the College of Physicians 6th September 1664." He (if the same William Swan) is also mentioned by J.H. Bloom and R.R. James, Medical practitioners in the diocese of London 1529-1725, Cambridge 1935, as, in his capacity as an apothecary, having certified one Joseph Freeman, chirurgeon, in 1692 (information from Paul Wheeler, Royal College of Physicians)