So my guess could be right after all... If this really is a Rheinmetall weapon, possiblity is that it was introduced to Shanxi alongside with its smaller caliber conterpart, the 10.5cm Type 16 and 8.8cm Type 18. As it is seldom mentioned in even the most comprehensive sources, its production number could be even smaller than that of Type 16 and Type 18(of which less than 100 were produced before the war broke out, some sources suggest), perhaps only limited to a few prototypes.
I agree with your view that the carriage may be the same as sFH 13.
As I have mentioned this is in a superb artillery exhibition hall in my city (Nanjing/Nanking) which only opens to the public on some special occasions. It last opened to the public a few weeks ago and I forgot to visit(shame on me!
). These photos were taken by a visitor and I found them in a Chinese forum, unfortunately he didn't take photos from the rear.
I'll try to arrange a personal visit to this exhibition hall this summer and I hope I can succeed. After all there are tones of interesting piece of artillery, although this 15cm howitzer is perhaps the most intersting.