diddley bow vs diddley-bow

diddley bow

noun
  • A one-stringed instrument (monochord) that consists of a wire string attached to something solid like the side of a house, and played with a piece of metal or glass, originating in the African American traditional music of Mississippi in the United States. 

diddley-bow

How often have the words diddley bow and diddley-bow occurred in a corpus of books? (source: Google Ngram Viewer )