I came across this item that I believe is either a CADs round or a small percussion detonator from an artillery fuze or similar. It is made of cast pot metal with a hole bored through the centre and a percussion cap pressed into the top. There is a screw thread at the bottom. It is 24.5mm (.960") high and 20mm (.780") wide at its widest point. The precussion cap is brass and is sealed with reddish purple annulus. There are no markings of any kind. See photo below next to 9mm Luger round for scale.
Once I get back to work I can measure the thread at the bottom, this may provide more of a clue about its origin. (Whitworth = UK, Unified = USA, Metric = Rest of world).