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Turning to one of my favourite hobby horses, currently advertised on eBay is yet another Spitfire altimeter. https://www.ebay.co.uk/itm/165432773091  It isn’t of course, but the dealer might not have been far from the truth if he had claimed that it might have been seen at the navigators Station of a Lancaster Bomber.  Prompted by a pointer from Mike Cain I went on to identify it as a Smiths MkXVIIA altimeter https://aeroantique.com/products/altimeter-mk-xviia-ref-6a-1538-0-35-000ft-british-royal-air-force?variant=42039497930. However, why no millibar/”hg setting; why a 0-20 face, not 0-10; why only one height needle; and why the coloured flag where the millibar value should be? 

Apparently, you turned the knob until a lubber line appeared in the little circular hole near 18,000ft, but what had you set then, 1013mb??, and why the colours in the window.  Was red a warning red?  E.g  if you set the height needle to airfield height on take-off and red appeared, did it mean that sea level pressure was less than 1013mb, so take care.  Any ideas or suggestions? DaveP