From an intriguing website, new to me, called Tubemapcentral.com.This map comes from a pdf newsletter available on the site. Best way to properly view it might be to download the pdf and then enlarge the map. Brings back a disappeared world. Not entirely forgotten, especially probably if you were a non-British resident somewhere far out in the empire.
The keeper of the newsletter writes:
“Many of you will be familiar with a particularly splendid poster from 1937 advertising air services by Imperial Airways. This included a schematic map in an inset, detailing mileages and frequencies of flights to all sorts of exotic destinations.”