number two vs side piece

number two

noun
  • mistress (the other woman in an extramarital relationship, generally including sexual relations) 

  • A somewhat small town, between a number one and a number three, where the success of a theatrical performance is unlikely to be very great. 

  • The batsman who opens alongside the number one, but does not face the first delivery. 

  • A pencil with hardness level 2. 

  • A first mate. 

  • Feces; the act of defecation. 

  • A primary assistant; vice-leader. 

side piece

noun
  • A mistress; a woman with whom a man carries on a physical affair (not emotional affair) without having the intention of forming a serious relationship. 

How often have the words number two and side piece occurred in a corpus of books? (source: Google Ngram Viewer )