postcard vs third class

postcard

noun
  • A rectangular piece of thick paper or thin cardboard intended to be written on and mailed without an envelope. In the case of a picture postcard one side carries a picture or photograph. 

verb
  • To send by means of a postcard. 

  • To send a postcard to. 

third class

noun
  • A reduced-rate mail service for printed matter. 

  • The cheapest accommodation on a train or ship. 

How often have the words postcard and third class occurred in a corpus of books? (source: Google Ngram Viewer )