Token, copper, 27mm diameter. London and Middlesex tradesman's ticket, 1795. (DH Mddx 293). Issuer J Henderson, filter maker, 1795. Obverse Filtering apparatus, ‘For purifying water, 1795’. Reverse ‘The Filtering stone ware-house, Coventry Street, London’. Notes Water was poured into a depression in the stone which was made of sandstone. The water percolated through slowly at the rate of about one gallon an hour.