hash mark vs steel square

hash mark

noun
  • One of the parallel lines on either side of the face-off circles. 

  • A service stripe, as on an enlistee’s uniform sleeve. 

  • An inbounds line. 

  • The hash or pound sign (#). 

steel square

noun
  • A tool used to measure out right angles. 

How often have the words hash mark and steel square occurred in a corpus of books? (source: Google Ngram Viewer )