The Benke-Thiemann Shoulder Stock in question was a post-ww1 commercial item... that was not issued to the military to the best of my knowledge... of course that wouldn't preclude a military person from buying and using their own pistol...
... the photos that I have (see below) of the device show a holster of much different construction...but perhaps someone more knowledgeable about this device can comment...
Here is a short article on the device which unfortuately, I don't remember where it came from other than someone sent it to me...Click on the thumbnail image to see it full size so you can read it...
<a href="http://boards.rennlist.com/lfupload/Benke-Tphoto3.jpg" target="_fullview"><img src="http://boards.rennlist.com/lfupload/Benke-Tphoto3.jpg" width="400" alt="Click for fullsize image" /></a>