I think that this elusive puzzle has already been discussed on Fifteen Squared, though that may be my memory playing tricks. No harm in trying again, of course. As far as I can see the Guardian archive starts at 1999, so it's unlikely the puzzle is available given the dates you mention. Google search produces nothing useful, as you'll have found out yourself.
The suggestion to contact the Guardian is a good one but from past experience, you may well be telling your disbelieving great-grandchildren stories about lockdown before you get a reply - if you get one at all. Good luck!