memorandum vs pro memoria

memorandum

noun
  • A short note serving as a reminder. 

  • A brief diplomatic communication. 

  • A written business communication. 

  • A page in an annual publication honoring the memory of a person who died during the past year. 

pro memoria

noun
  • A memory-aid; a reminder or memorandum, especially a book or document serving this purpose. 

  • A document circulated in a meeting or summit for discussion purposes. 

How often have the words memorandum and pro memoria occurred in a corpus of books? (source: Google Ngram Viewer )