Air Ministry grid navigation compasses

Here are some more sources of information.

There have been quite a few threads on related topics, try searching for thread 113073 “Compasses and Alcohol”.

Hope these help, essentially the number gives the diameter! I have some more possibly useful info if you PM me with a conventional email address.

I suggest a useful starting point could be in one of the (many) volumes of AP 1086. 
 

Then there is the ever useful https://compassmuseum.com/aero/aero.htm that suggests AP 1275 too.