Here the political opponents of Sir Robert Walpole met, using the title of the Fountain Club. Since the Kit-Cat Club, Walpole's supporters, also met here, we have to trust that the landlord arranged for the two clubs not to clash; perhaps the Fountain Club met on Fridays and the Kit-Cat on Saturdays.
Lawrence Silverman adds: The Fountain Tavern was also where the Whig party leaders met to plot the Glorious Revolution of 1688/9 which deposed James II, enthroned William (of Orange) and Mary, made Parliament supreme and gave us the Bill of Rights.