post office vs third class

post office

noun
  • A place (building, office, shop, or counter) concerned with the business of delivering letters, post or mail and selling stamps, etc. 

  • A party game involving the exchange of kisses. 

  • An organisation that delivers letters. 

third class

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

  • The cheapest accommodation on a train or ship. 

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