Ammunition manufacture requires lots of gauges to make sure manufacturing tolerances are kept. This is a strange one which so far defies identification. It measures 13mm at the rolled mouth,14mm just above the rim. heavy wall tinned steel. headstamped by 2 Czech arsenals,serial numbered. Solid head.gyu (Gebruder Michera, Prague), dov (Waffenwerke Bruenn, Wsetin plant, Czechoslovakia) . ???