This is the Ball Rohr hand grenade, also once attached to a cardboard handle, remnants of which you can still see inside the ball.
This grenade was not a replacement for the previous Rohr, which was a concussion grenade, designed to kill through blast,…
…but, as you can see from the small shrapnel fragments that still survive set into plaster inside the head, this was a fragmentation grenade, designed to spread shrapnel across a wide area, and thus used in entirely different situations to the concussion Rohr.
The cardboard handle went right through, extending a short distance from the top of the ball, but, as with all cardboard-handled grenades, was prone to disintegrate in damp conditions.
The body of this grenade was made of cast iron, and sensibly a small section of the ball was flattened so that the grenade would be stable when put down and would not roll around, as balls tend to do.
Once upon a time a belt handle was welded onto these two small protuberances,…
…as the two insets top left show (on the left a replica, on the right possibly an original). Bottom left; Austrian sentry with Ball Rohrs ready and waiting. Top right: Austrian soldier practising throwing a Ball Rohr in early 1917. Bottom right: Austro-Hungarian troops with Ball Rohrs advancing carefully through barbed wire on the Belorussian front.