Soviet Union vs Union of Soviet Socialist Republics

Soviet Union

name

  • The Russian Soviet Federative Socialist Republic. 

Union of Soviet Socialist Republics

name

  • A nation in Eurasia that existed from December 30, 1922 to December 26, 1991. 

How often have the words Soviet Union and Union of Soviet Socialist Republics occurred in a corpus of books? (source: Google Ngram Viewer )