Original German practice dummy stick grenade. This is made out of wood and metal. These were essentially used to replicate the throwing of a stick grenade so that soldiers could practice attacking various positions. For example, when an attack on a fortification was to take place, a replica of the fort would be set up and the unit tasked with attacking it would train by attacking the replica. So you would use a training grenade like this to repeatedly throw grenades at specific positions to get your throw correct. Like knocking out a bunker. That is also why this and others like it usually appear very well used. This one is actually in rather good condition. It is a bit rusty, but its not all beat up and dented.
I had a great deal of fun with this. I feel these practice grenades are an underrated piece of history. This is not to be confused with other types of training grenades which were meant to familiarize people with the operation of using a stick. This was purely to practice throwing a stick, so you could put them where you wanted them. It would be safe to assume that many German soldiers were more familiar with throwing one of these than the real or training versions.