canister vs letterbox

canister

noun
  • A cylindrical or rectangular container usually of lightweight metal, plastic, or laminated pasteboard used for holding a dry product (as tea, crackers, flour, matches). 

  • A projectile component containing colored or screening smoke or riot control agent composition. 

  • A component of canister-type protective masks containing a mechanical filter and chemical filling to filter, neutralize and/or absorb toxic chemical, biological and radiological agents. 

  • Part of a windmill that connects the sails to the windshaft. 

  • Any of various cylindrical metal receptacles usually with a removable close-fitting top. 

  • A special short-range antipersonnel projectile consisting of a casing of light metal, loaded with preformed submissiles such as flechettes or steel balls. The casing is designed to open just beyond the muzzle of the weapon, dispersing the submissiles. 

verb
  • To pack into a canister. 

letterbox

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

  • 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. 

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