letterbox vs mail slot

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. 

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

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. 

mail slot

noun
  • A slot in a wall or door through which mail is delivered. 

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