Air Ministry , RAF Altimeter  type 6A / 685 

Vintage, probably WW2 era as its has been in the family since the 1940s. 

Diameter 82mm (3" approx) length 100mm (4" approx)

Probably a Mk 14 that was used in the following aircraft types.
  • Spitfire 
  • De Havilland Mosquito
  • Avro Lancaster
  • Avro York (C.1 variants)
  • Hawker Hurricane 
  • Avro Lancaster

  • The altimeter is belived to be working but accuracy is unkown.  It does change with changing air pressure