are sure that is 120mm DM13?
it's look like 105mm APFSDS
http://www.bocn.co.uk/vbforum/threads/100774-german-105mm-APFSDS-for-105mm-smoothbore-gun
btw can you measure DM43 length from tip to tail ?
Definitely 120mm.
Size comparison with a 105mm M735:
The DM43 is 71mm long. I am not sure it is an actual DM43, might be a test model.