For setting carburettor mixture on petrol engines. Correct mixture should show the combustion flame as blue. Plug has a glass window which allows you to see the colour of the combustion inside the cylinder head.
Doubt whether it has had much use. Belonged to my uncle, clearing some of his remaining stuff.
All appears to be there, plus the user manual, 18 pages A5 size on how to use it.
Please study all the photos.
In its original plastic case and transparent cover.