Altimeter, Mk XVIIA, Ref 6A/1538 0-35,000ft, British Royal Air Force

Here is a British Altimeter Mk XVIIA, Rf 6a/1538 as used in WW2 RAF bombers such as the Avro Lancaster, showing a maximum altitude of 35,000 ft.  

The altimeter has the marking of ICAN, the International Commission for Air Navigation. ICAN was an international governing body established on the basis of the ICAO relating to the regulation of aerial navigation and dealt exclusively with the regulation of international air navigation and in particular public international air law. The Convention came into force on 11 July 1922. Although in law the ICAN was placed, and remained, under the direction of League of Nations, in practice direction was replaced by friendly cooperation. An altimeter designed to ICAN specification assumes a constant temperature of 15 deg C at sea level and that this temperature decreases by 1.98 deg C for every 1000 ft increase in altitude. 

As the aircraft climbs, the pointer rotates clockwise indicating altitude along the outer perimeter scale.  When reaching the 12 o'clock position the aircraft is now at 20,000 ft, and the altitude is now indicated along the inner scale up to 35,000 ft. 

The knob is to adjust for barometric pressure, which will rotate the pointer and align an indexing line within the small round window located on the face at about the 18,800 ft marking. This knob is very stiff and could use a bit of TLC.