Great infos, those pictures are absolutely stunnig! Sorry I missed out so long but now I´m back home.
The Reichsarchive book 11 lists the Württemberg 4. "Tragtier-Kolonne" which would translate into 4. mounted carriage company (supposing a Kolonne is company size). A Tragtier is a carriage horse or mule, the German and Austrian armies still have Tragtier companies today in their mountain divisions. I could not determine which division this unit was attached to, but I guess its a good candidate for the helmet with the empty scroll.
Robert somewhere you determined the nationality of many of these GMGAs. where is that?
That was the book "Die Württ. Gebirgs- und Sturmtruppen im Weltkrieg 1914-18" by Hubert Lanz in 1929. Here is the info given there:
Württemberg: GMGA 250
Saxony: 249
Bavaria: 206-209 (later GMGA 262), and 248
Prussia: 201-205, 210-247, 251-255 (in 1918, 15 of these these bacame the new units 260, 261 and 263-265)
why don't you try to figure out what unit that 18 is
If the Württemberg troops that were sent to the Serbian front in 1915 used the scroll helmet for their carriage unit, it may have been used in other auxillary units too. I have to get back to the library my next free day and look through the references of the mountain troops in that book. For example, I could imagine the Gebirgs Artillerie-Abteilungen using scroll helmets too. But on the other hand, this 18 need not be linked to the other units in Serbia at all. Also note that the neck flap as well as the strap are different from the typical style that we know ftom the GMGA pictures.