The base has on the outer 'ring' "1917" - it also has " 26 12-17", the letters "FVO" and " 18 PR" with a roman "II" beneath it.. The letters "C F" appear to be stamped above the date (1917)
The inner ring has "S.H. & S" (which I take to be a reference to Samuel Heath and Sons), and also "4/27 LOT" with two arrows underneath. There are some other markings including "No 1" with "II" underneath and another small arrow.
I would be very grateful for any advice at all on this - it is highly valued (for family reasons) and I am sure that there is an interesting story behind it - knowing more about what it actually is would be interesting.