Mickey Mouse - For EXTRA Service ...

23.5" diameter sign printed on heavy gauge galvanized steel ... a specific steel that was created to accept this type of printing ink at the time (1940). Created for the marketing of Standard Oil Company of California's Standard Gasoline. According to one source it was used on the back of taxi cabs during the winter-spring season. Down at the bottom directly at "6:00 o'clock" is the familiar "notch" in the sign in which marketing signs would be placed into a frame that would force the sign to be straight and even. Then the whole frame would be placed on the back of a taxi.
Another source said they were covers for spare tires on commercial vehicles.