I came across this item that I believe is 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 (fired on this example). It is 24.5mm high and 20mm wide at its widest point. The precussion cap is brass and is sealed with reddish purple lacquer. There are no markings of any kind. See photo below next to 10p Piece for scale.