butterfly clip vs cotter pin

butterfly clip

noun
  • Synonym of split pin (“kind of stationery fastener”) 

  • A hairclip shaped like a butterfly, usually made from brightly colored plastic. 

cotter pin

noun
  • A split pin, the ends of which project through an object being fastened and are bent back to hold it in place. 

  • A pin or wedge inserted through a slot to hold machine parts together; a cotter. 

How often have the words butterfly clip and cotter pin occurred in a corpus of books? (source: Google Ngram Viewer )