I got this grenade from Czech Republic, but it seems uncertain where it originally comes from.
On the inside there is a small pin fixed in the centre to the solid base. It is as thick as an average nail.
This pin goes up half-way inside the grenade.

Length: 87mm , Diameter: 54mm.
Weight: 625gram.
Diameter hole in top: 15mm

Darryl came up with some information.
Apparently this grenade was designed in the 1920's, but it was not adopted as a service grenade.
Enough bodies had been produced before this was decided and the grenades were degraded to throwing dummies.