MX vs atomic bomb

MX

noun
  • on early British stamps, Maltese Cross cancellation mark. 

atomic bomb

noun
  • The possession of one or more atomic bombs; the ability to launch a nuclear attack. 

  • A nuclear weapon that obtains its destructive energy from nuclear fission. 

How often have the words MX and atomic bomb occurred in a corpus of books? (source: Google Ngram Viewer )