The theme is Painting By Numbers, and the posters featured (just 20 of the possible 500 the museum have in the archive) are all what we'd now call "infographics".
The museum refers to the posters as "data visualisation", and the works on show from the 1920s and 1930s onwards.
They're also very beautiful pieces of design.
Like the poor guys harvesting "rubber" in this one:
|Figures for 1923 by Charles Shepard (1924)|
And this one, simply called "Speed" is from a decade earlier, but is so fantastic in terms of getting the message across. (I wonder what the average speed of a London bus is today...)
|Speed by Alfred Leete (1915)|
Again, there are gorgeous details: the monkey travelling at 2mph, the wonderful silhouetted figure of the lady on the same row, the dog racing ahead of that car travelling at 12mph...
And here's a really 60s effort from Heinz Zinram. That's a more recognisable London Transport kind of font right there...
|These Vehicles are Carrying 69 People, by Heinz Zinram, photographer (1965)|
Here's my original post on londonist.