I remember seeing this some years ago and thought I'd taken some pics but they seem to have been lost or misplaced, anyway I visted again last year but the gun was gone...
Indeed it is a 9cm c73/91. Though obsolete, these systems saw use after the expansion of German aritllery batteries during the war. Served primarily by reserve foot artillery personnel.
This gun is a "ringkanone" - reinforcing tube in front of the breech. I agree it's a C73.
The C73 and the very similar C80 saw a lot of service with the Ottoman Army before and during WW1. There were large purchases of the C73 (489) and C80 (404)
in the 1870s and 80s. The Turks liked the gun because it was simple and reliable. The Imperial Arsenal in Istanbul built a fair number (130?) under licence. There
is some evidence that the C73/80 was still in service in the War of Independence (1919-1923).