Released as an online tease a couple of years ago by CBS Outdoor, evidently there are 75 different London Underground stations represented in there. Even though it’s been around for a couple of years I still can’t seem to find a definitive list of all the stations anywhere – perhaps you can still win prizes.
The one below is version two. This evidently has 100 underground, overground & DLR stations cryptically represented.
Here’s a detailed map that shows all the stations in the London area.