Eine kürzlich ausgegrabene 305-mm-Mörsergranate aus dem Ersten Weltkrieg, in der Nähe des Parlamentsgebäudes in Belgrad

Most likely a 285kg shell, as the 305mm ones came in 280-380kg range.

Pictured:

1) Serbian police measuring the shell prior to demining

2) A Škoda 305mm siege mortar at the Belgrade fortress which would have fired the round

3) Highlighted is the Assembly building (under construction) in 1914.