It is incredibly difficult to get individual details from the German authorities. Even their roll of honour is difficult to use.
Basically, like for British personnel, I would say that at this time it just has to be accepted that it belonged to a serviceman, and hope that more of the files will be available in the future. But a lot of the German files were destroyed during and just after the war.
However, if you want to know about the dagger then there are very knowledgable people on
Axis History Forum :: Index who could help you with dating it etc