post-obit vs postmortem

post-obit

noun
  • A bond in which the obligor, in consideration of having received a certain sum of money, binds himself to pay a larger sum, on unusual interest, on the death of some specified individual from whom he has expectations. 

postmortem

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