HI Bombas,
it's much likely a Granatschrapnell (shrapnell with high explosive head part). Did you meassure exact diameter? And also counting the number of grooves in the drivingband can help to identify the gun.
Such austrian Granatschrapnells are known in calibres 7,5 cm, 8 cm, 9 cm, 10 cm, 12 cm, 15 cm, 30,5 cm, and 38 cm but not jet in 21 cm. But the documents are incomplete and on Gyr firing range also trials are much possible. Most large austro-hungarian guns were tested on Gyr range.