Beautiful Luger & rig and great story to go with it!
Those grip strap markings are a neat piece of WW1 history.
There are many variations and as many interpretations at times as the field armorers that applied them didn't always follow a set pattern of characters.

Yours could be 2nd Machine Gun Co., 15th Regiment., Weapon #8
..or another interpretation.
Maybe 1st Regiment Weapon #58....or....
Issue handguns in the field usually had a 'Weapon #' at the end of the stamping,,and not always clearly separated from a Co, Squadron, Battery, ect number.
The Germans were very meticulous about numbering and keeping track of things.
Figuring out what unit it is can lead to some interesting historical info on the group sometimes thru research.

Looks like an original marking was removed under the '158',,then 158 applied.
You can see some crosswise file/polishing marks and a few bump up marks from the inside of the grip strap just below it.

It looks to me like the 158 numbers have been hand engraved, probably hammer & chisel, rather than stamped into the metal. Maybe they didn't have stamps available in the field and a decent gun mechanic can make a simple graver easily enough and cut numbers like this. The 5 and 8 are deffinetly not stamped,,neither is the bottom serif on the '1'. Just an observation..


Not uncommon to have these markings changed in service as the pistol got around. Many simply had the old marking struck out with a single line or X's and a new marking added.
Some had the old marking or part of it more tastefully removed and new digits restamped.
The grip strap is very thin in the front and seeing the metal collapsed a bit is not uncommon either from the unsupported hammer strikes of the stamps.

Great pics of all the Lugers on the thread! A favorite of mine for sure..