Hello Klaus!
It´s hard to say something about the stamp. Fact is, it is a German, not an Austrian one.
It´s a so-called "blind stamp" with no info on it but the date.
The troop-stamp shows us the GRP-Regiment, not GRP-Bataillon.
There is no company marked, so the sender was in a company that was not detached to another unit. So the letter was sent by the regimental staff.
The battalion was raised into a regiment in April 1916. The skull head was introduced end of July 1916
So we have August 16, 17 or 18.
I coulnd´t see, the regiment was in the East in 1916
Probably we are in 1917 an we have the above mentioned theatres:
From Febr.20, 1917 until the end of 1917 the 3rd company had two actions in Russia and 1 in Galicia.
The 4th company 1 action in Galicia and 5 in Russia and the 7th company 4 times in Russia.
In Rumania, Macedonia, South-Tyrol and Italy:
8th company: 3x Rumania, 2x Macedonia
Platoon Hübner: 1x Tyrol, 2x Italy
Platoon Hahn: 1x Tyrol
I also wrote above:
Since Jan.5, 1917 the Eastern Front was divided in Heeresfront des "Oberbefehlshabers Ost" (Commander-in-Chief East) and the Heeresfront Erzherzog Joseph.
The Heeresfront Erzherzog Joseph consisted of:
K.u.k. 7th army, K.u.k. 1st army and Heeresgruppe Mackensen with german 9th army and bulgarian 3rd army.
So I am sure, the card goes to the german 9th army.
17.8.17:
26.7.17-9.12.17: Trench-war at the Sereth and Susita (Rumania)
6.8.17-3.9.17: Break-through-battle at the Putna and Susita (Rumania)
So I assume, the card was written in France to the 8th company of that regiment in Rumania.
In August 1918 I have no notification in the East.
In the West the different companies fought at different places in that era.
By the way...I found Billerbeck as a Leutnant in the regimental history!
Unfortunately no info, in which company...
That means he promoted to Officer later!
I checked the "Militär-Wochenblätter":
Schlodder: No entry. So he was only Offizier-Stellvertreter with the official rank of Vizefeldwebel
Billerbeck was promoted from Vizefeldwebel to Lt.d.Res. April 27, 1918.