The story as I first heard it starts with a short Notice in the 'Orders and Notices' manual which everyone was supposed to read before flying.
In those days we always referred to the BOAC HQ at Heathrow as 'The Kremlin'. You know the sort of thing - a new instruction to crews is published and crewmembers in hotels and bars down the route would say things like. "Those bl**dy blokes in the Kremlin, can't organise a piss up in a brewery, etc, etc."
So a notice appears saying something about, 'when staying in Moscow, it would be advisable not to make disparaging remarks about the Kremlin, the rooms being bugged and the possibility of the listeners misinterpreting the usual crew chat.'
Naturally, crews being only human, they search their rooms for bugs. Pictures are taken down, telephones dismantled, chests of drawers examined, bed knobs unscrewed, etc, etc.
Then, when staying in Moscow, they all gather at the crew party in the captain's suite, and everyone searches for bugs. A slight bulge is noticed in the carpet, and it is rolled back to reveal a sinister looking fitting with a large nut in the middle. So the engineer is sent off to his room to bring back his box of spanners (all flight engineers being well equipped). He returns and unscrews the nut, at which point the central bolt disappears followed by a loud cash from the room below. And then it dawns upon them.....because exactly above their heads there hangs a chandelier!
So the nut is carefully replaced, the carpet rolled back, the party continued and not a word is said.
Well....that was how I heard the story! I guess there are many versions.