This map is hanging in a Minnesota college. Its a colorprint map from around 1952 with certain countries shaded in green. The shading does not seem to be random distribution; all of Southeast Asia is shaded as is almost all of Europe. Cambodia is entirely surrounded by shaded countries so, if this were a matter of alternating, it should be white. My question is what is the pattern of shading for this map? What unites all of the shaded countries? What is this map for?

Here is the map in question: http://imgur.com/a/MkvtS