Since 1927, several British newspapers have organised competitions challenging their readers to identify mystery men in order to claim prize money—cf. history of the British name ‘Lobby Lud’ and history of the British name ‘Chalkie White’.

In the summer of 1983, in The Guardian (London and Manchester, England), the British journalist Alan Rusbridger (born 1953) organised, as a prank, a one-off competition challenging the readers to identify a mystery person in order to claim a £10 book token.

He launched this competition in his column Diary, published in The Guardian of Tuesday 23^rd August 1983:

Now, here’s your chance to do something other than pontificate. This column is not proud. It realises that August is a slack month and that many of its regular readers are off pontificating in sunnier climes. So, in the honourable tradition of Lobby Lud, Chalkie White and London Standard’s “Standard Man” the Diary today lets loose its very own Mystery Person.
Here, you see Mystery Person’s eyes. The person is male and at all times carries a copy of the Guardian in his right hand jacket pocket. Today his route will take him through pontificating Guardian-reading-infested areas of London. Between 12 and 3 pm he will take in the headquarters of the NCCL¹, the SDP² and Channel Four³, as well as the Barbican⁴, the Home Office and the Number 73 bus through Islington. Stop him, say: “You are the Diary Mystery Person and I claim my £10 book token” and a £10 book token will be yours. One prize only. Result tomorrow.

¹ NCCL: National Council for Civil Liberties
² SDP: Social Democratic Party
³ Channel Four: a media company established under the Broadcasting Act 1981, and originally provided for by the Independent Broadcasting Authority
⁴ the Barbican Centre: an arts and learning centre in London

In The Guardian of the next day, Wednesday 24^th August 1983, Alan Rusbridger lampooned the readers but gave them a second chance:

What a feckless lot! Too busy pontificating, obviously. Not one of you managed to spot the Diary’s Mystery Person on his three-hour trek through Guardian-reading London yesterday. For 15 minutes he stood outside SDP headquarters. No one came in or out (All in Tuscany?). For 10 minutes he loitered outside the Home Office. A policeman asked him to move on. Channel Four . . . the NCCL . . . the Number 73 bus through Islington . . . it was all the same.

We will try again today. My man will be outside the main gate at BL, Cowley, intent on flushing out any remaining moles, on the basis (qv last week’s Sun) that they all read the Guardian⁵. Here is one more facial clue and he will have a Guardian in his right-hand pocket. Walk up to him and claim your £10 book token. PS: non-moles may also apply.

⁵ A Trotskyist organisation had allegedly infiltrated the British Leyland (BL) factory at Cowley, Oxfordshire, and, on Thursday 18^th August 1983, The Sun (London, England) published a guide on how to spot “moles” on the shop floor—guide which contained the following (as quoted by Alan Rusbridger in The Guardian of Friday 19^th August 1983):

You never see them reading the Sun. Their favourite paper is usually the Guardian or the Financial Times.

In The Guardian of Thursday 25^th August 1983, Alan Rusbridger announced a winner and detailed the mystery man’s forthcoming assignment:

Caught at last! After three hours of wandering unspotted through some of the most fertile Guardian-reading terrain in the country, the Diary’s mystery person was nicked within 2½ minutes of positioning himself outside the front gate of BL Cowley.
That is the good news. The bad news is that the mystery person was, alas, not identified by a BL mole, which was the intention, but by a local GP, who had spent a long time driving past the gates waiting for my man to turn up.
To make matters worse, Dr Matthew Wilkinson (for it was he) at first accused my man of being Chalkie White. To compound matters, he then told a very bad joke: “The only moles I see are black and on people’s backs.”
Dear me. Congratulations, anyway, and a £10 book token.
Ironically, my man had already been spotted on the train to Oxford by Mr Keith Dickinson, a member of the editorial board of Militant. On being told he could not have the prize, being on a train rather than at Cowley, Mr Dickinson proceeded to lecture my man on the nature of moles.

For his final assignment today, Mr Mystery is being pitted against his so-called rival, the Standard Mystery Man, a vulgar, double-barrelled character wont to flash large sums in used fivers. My man will dog the footsteps of the Standard man as he makes his way up and down Piccadilly today and has strict instructions to find him before he is found.
A final picture clue: a Guardian in his right hand pocket. All yours.

Finally, on Friday 26^th August 1983, Alan Rusbridger gave an account of the last episode of The Guardian’s mystery man’s adventures:

And they say dog doesn’t eat dog! If you can believe it, the Diary Mystery Person was rudely awakened yesterday morning by a reporter and photographer from the London Standard, demanding his name, snatching his photograph and threatening to nobble him and his attempt to identify the Standard mystery man in Piccadilly later that day.
My man is nothing, if not resourceful. He gave a false name and went off in search of a false beard. But no sooner had he arrived in Piccadilly than two of Lord Matthews’s overpaid newshounds and the same weazly [sic] photographer started assaulting him and claiming their £10 book token.
For three hours they trailed him, repeatedly failing to utter the required words in the required order and feebly spurring innocent members of the public to behave likewise. They then slunk off to telephone a sadly distorted and garbled version of events to their “newspaper.”
In between these diversions, my man was stopped around 20 times, but at least 14 of these mistook him for the Standard man. The closest to winning was a young girl, who was beaten around the head with a newspaper by her mother for suggesting that she had spotted him. Sadly, no-one came up with the required wording and the book token will remain in the Diary safe for another year.