Now that I look back on it, the two Linleys in the 1774 are differentiated by an additional mark on the pipe (neglecting the fact that that mark is in the wrong orientation). John Linley has an additional diamond. However, the only listing in the 1787 G&M is John, but here without the diamond (William's mark from the 1774). It is now Lindley rather than Linley. More research is clearly needed...