Thanks for your words!
It could be an 88 or an 86 as it's faded, but the 86 fits too, interesting info.
I'd love to track the owner but I cannot read it in person either. If you want I can make another pic of it but I doubt we will be able to read it.
There's what looks to be the name and surname (the name starts with L and the surname with K, both are very short so maybe they are something else) and below it a "476" or "496". Above the "name" there are other numbers, but they are even more faded.
EDIT: I looked up more information about RIR 86, it's third batallion was organized in Schleswig and the 1st and 2nd in Flensburg (mine is from the first batallion apparently).
The regiment's organization was completed on August 9, 1914 and was deployed to the south of Flensburg to serve as a border guard against the danes until August 22-24 of the same year, when it was mobilized for the invasion of Belgium and detrained in Esemael. It also fought in the early battles of 1914, like RIR 88.
The regiment was part of the 35. Infanterie-Brigade, 18. Reserve-Division, IX. Reserve-Armee-Korps. It's strength upon departure was: 83 officers, 3190 other ranks, and 240 horses (source: Wikipedia).
Here I found a list of RIR 86 officer's, sadly no ORs: http://www.denkmalprojekt.org/2018/vl_reserve-infanterie-regiment_86_offiziere.html
And also this, which has a lot of info on the regiment but it's in danish (it's a translation of the book "Geschichte des Reserve-Infanterie-Regiments Nr. 86 im Weltkriege ", written by Leutnant d. Res. Friedrich Khlahn in 1925): https://denstorekrig1914-1918.dk/wp-content/uploads/2016/10/RIR86_dansk.pdf
And a picture of members from that regiment wearing mützen (c.1917 because of the officer's cap and M15 jackets, but atleast one of the ORs still wears the older cap I have), for sale: https://picclick.fr/CPA-PHOTO-Soldats-allemands-du-Reserve-Infanterie-Regiment-123118502589.html
Another pic: https://picclick.fr/WW1-German-Army-Schleswigsches-Reserve-Infanterie-Regiment-Nr-86-Used-401689531053.html#&gid=1&pid=1
EDIT 2: Looked at it with a magnifying glass and natural light and it's an 86 indeed. Still can't read the writings. Thanks for your help!