Unknown Vintage Cased Military Marked Metal Spring Disc

To be honest I have not got a clue what this item is so I will tell you what I know, this item comes in a lidded container, it is made from metal with a large round spring underneath which is half an inch deep, there is a hole in the centre which has a screw thread, on the top it has the broad arrow mark and a few numbers and letters, it has been kept oily and there are two grooves in the edge, it measures two inches in diameter and is solid and heavy. Maybe mechanical or for photography, perhaps if you know you can drop me a line.

Code: 29878