Note that the original poster is not from the US, so our laws don't count. If laws are ignored (or the OP is in an official capacity) in the US, the Swiss Furrer would be the rarest, since few have been released and none were sold on the surplus market or brought back as war trophies. Most of the others are scarce, but not as rare. The Thompson is the most common in the US (thousands were sold as DEWATS, then opened up during the 1968 amnesty); the German MP 38/40's are also fairly common, being a favorite war trophy.
Jim
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