The Number 239 is our second route which does not exist any more. This is because, in February 2008, it was amalgamated with the 170. See here for many details.
Strangely, the 239 had had many incarnations before it died, starting in Essex in the 1950s, running between Romford and Gidea Park until 1958. Then it was reborn in the 1960s, to replace a trolley bus (the 639) between Hampstead Heath and Finsbury Square, only to stop in 1970
Then the 239 reappeared, running between Tufnell Park and Aldwych, though it stopped at King's Cross on Saturdays and did not run on Sundays. That lasted, with a few variations, until 1982, when the route was withdrawn.
Between 1990 and 2008, the number was reused for the Roehampton to Clapham Junction journey. I should not like to say that it will never return, however we do feel that we have already 'done' it, as we rode the 170 all the way from Victoria to Roehampton on 9 May this year.
To think we thought bus numbers were immutable, other than a lengthening or shortening of a route: this one - Essex, then NW London, then SW London, proves how wrong we can be.