billfold vs kit violin

billfold

noun
  • A small, folding sleeve or case designed to hold paper currency, as well as credit cards, pictures, etc. 

kit violin

noun
  • A very small long-necked violin that came in a variety of shapes and configurations, meant to be carried in a pocket and intended for dancing instructors to carry to accompany their students. 

How often have the words billfold and kit violin occurred in a corpus of books? (source: Google Ngram Viewer )