Before the decisions of the 1952 Brussels UPU congress came into effect on July 1, 1953, aerogrammes (or: air letters as they were named at that time) were no standard UPU postal items. Countries had to make bilateral agreements about accepting aerogrammes at, compared to ordinary air mail letters, reduced rates in incoming mail.
Here is a (private) cachet commemorating the start of air letter services between Palestine and the US on May 15, 1947.
There is a matching item from Australia elsewhere on this blog: http://aerogramme-airletters.blogspot.de/2015/03/australia-start-of-air-letter-service.html