pennywhistle vs tin whistle

pennywhistle

noun
  • A six-holed flute-like instrument with a fipple. They have approximately a two octave range (sometimes a little higher). Stereotypically, they are made out of tin, but in reality they come in all sorts of varieties, including tin, brass, nickel, cane, polymer, etc. 

tin whistle

noun
  • A small, high-pitched metal whistle with six holes that is blown using a fipple. 

How often have the words pennywhistle and tin whistle occurred in a corpus of books? (source: Google Ngram Viewer )