This is a reuseable practice grenade made of cast iron. After use, a new fuze was installed.
Markings : ÜB HGRK DM 48
The letter "K" probably stands for "körper", body.

Length w/fuze : 109mm
Length w/o fuze : 86mm
Diameter : 58mm
Hole in base of body : 15mm
Empty weight : 492gram
Fuze :

Photos below © Geert.

The DM-48 shown right and below is in my own collection, and is different from the one shown by Geert, above.
Not only the markings are slightly different, but also the base , which seems to be made to fit a cover.
This type seems older then the "K"-marked one. The red gasket between fuze and body was put there by me.
Markings : ÜB HGR DM 48

Length w/fuze : 110mm
Length w/o fuze : 86mm
Diameter : 58mm
Fuze :
DM-32 (it fits, but it is unsure if this was used with this DM-48).