The introduction of the tuppenny post in 1801, together with the addition of the poet Scott (who only became popular with the publication of his The Lay of the Last Minstrel in 1805) to Cowper and Thomson, is apparently the marker used to date revisions of S&S, according to something I read only a couple of days ago but already I can't remember where.

Getting old - it's a mug's game. Don't try it!