As I said, in my region, there is a great deal of forts built by German before the Great War....
Below, a German map with the position of these forts, as well as their German names.
Forts depended on regiments of infantry as well as regiments or company of machine gunners of fortress (FMGA, festung maschinen gewehr abteilung) and (FMGK, festung mashinengewehr company)
The forts of Metz depended:
FMGA 12 to 15
Regiment of infantry: IR 98, 130, 144, 145.
Photo of the special knot for the 14th compagny, of infantry regiment. Its' the compagny of the fortress machin gunner (in white, beggining of war and in grey, middle since end of war)
Machine gunners' dog tags of fortress of Metz
This below, in the fort of Mutzig in Alsace...

I also have the 2 legs but it's difficult for showing the mount...
This below, nowadays, it stays a lot of MG's binker mount in position; the top was destroyed by scrap merchant after the war.
I hope that now, you can better understand what is a forteress mount...
Best regards
david