field boot vs moon boot

field boot

noun
  • a type of walking boot 

moon boot

noun
  • A type of footwear made popular in the 1970s, with a thin rubber outsole and no distinct left or right feet. 

  • A large hard plastic covering, used to protect a broken foot or ankle while it heals. 

How often have the words field boot and moon boot occurred in a corpus of books? (source: Google Ngram Viewer )