diddley bow vs diddly 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. 

diddly bow

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