letterbox vs post box

letterbox

noun
  • A delivery point for mail, a box, compartment or slot for this purpose. 

  • A hidden container that holds a logbook and rubber stamp, found by following clues as a form of recreation. 

  • A rectangular hole or slot. 

  • A collection point for mail intended for onward delivery, a secure box or receptacle for this purpose. 

verb
  • To transfer a widescreen motion picture to home video formats while preserving the original aspect ratio, with the placing of black bars above and below the picture area. 

  • To hunt for letterboxes (containers with logbook and rubber stamp) by following clues. 

adj
  • Transferred to home video formats while preserving the original aspect ratio, having black bars above and below the picture area. 

post box

noun
  • A box or slot into which a courier or postal worker puts letters for a recipient to collect. 

  • A box in which post can be left by a sender to be picked up by a courier or postman (postal worker). 

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