I am by no means a Luger expert, so hopefully someone here knows:

My best friend’s father recently passed away suddenly at his home. He was a WW2 vet and had a small gun collection. My friend knows his dad had a postwar P38 and a matching P08 1940 Luger in it’s original holster with extra clip & mag loader. His father kept these 2 pistols in his desk.
Well, the neighbors reported the death, and between the time Law Enforcement/EMS arrived, it seems that several neighbors entered the house to "help out".
Anyway by the time my friend drove the three hours it takes to get there, the two pistols vanished, along with an undetermined amount of cash, leaving only the Luger holster with its spare clip & mag loader.
No rifles or anything large was taken, leaving my buddy to believe a neighbor took them while in the house. his dad did not get along with some of his neighbors.
My friend told the police, who requested serial numbers.

The original Walther P38 box with test target, sales receipt, serial #, etc was found so he has the information for it.
The Luger, all he has is a 4 digit serial number that was written down, spare magazine, holster & mag loader?
How else would a 1940 Luger be marked so that he can properly report it to the police?

Here is a picture of the back of the holster if it would help:

Thanks for any help with this, my friend is having a hard time with all of this.

Click to enlarge the picture