This was an attempt a few years ago to try to get ordinary tube passengers playing Mornington Crescent and spreading the game far and wide. To this effect, LU commissioned Stovold Ltd (a company set up by the trustees of Mr Stovold's estate) to produce a set of posters to help explain things to bemused passengers.
After much research I have found one of these posters!
I have no idea when it was printed. I may have to contact Stovold themselves and ask them but it just goes to show that despite LU's good intentions, the idea never really caught on and was abandoned after a few months.
Here's the scanned poster:
