A decedent who was a citizen of the United States and a resident of a possession thereof at the time of his death shall, for purposes of the tax imposed by this chapter, be considered a "citizen" of the United States within the meaning of that term wherever used in this title unless he acquired his United States citizenship solely by reason of (1) his being a citizen of such possession of the United States, or (2) his birth or residence within such possession of the United States.