Luger pistol DWM 1918 New luger found in holland

[mercedes200]

This pistol has found by someone at home in the Netherlands a sort time a go. I hope, that one of you can give of any information of this weapon.
The serial of it is 3515, I forgot to note the suffix but I give you this later
if it is necessary.
It has also a regimentâ??s number on it on the metal beyond the grips on the site of the trigger. The number is: R.S. 4.19
The two magazines ware loaded whit ammo from WW1.
The ammo was dated 1914-1915 and eats clip has 5 cartridgeâ??s

The luger is in good condition and all of the numbers match.

Question: whitâ??s regiment was this?
and what is the prize of this luger when I sold it?