How has this survived? It was in 1968 that there was a further Clean Air Act so this has been in place for at least 43 years. Why are some of the letters visible and legible (you can see 'authorised')? Did the ink from the poster seep into the paste or glue used to put the notification up with?
Clean Air Act 1956 - Wikipedia, the free encyclopedia
You can see the 'ghost notification' on the corner of the building at about shoulder height