Hello Charley,
what I have found:
I found a drawing of a german "15 cm Gr. 12 mit russischer Führung" (15 cm Gr. 12 with russian type driving band) in a german archive and also a "15 cm Gr. 14 m. russ. F." mentioned in the text.
In a different correspondence about planning the ammunition production for foreign guns a "russische 15,24 cm Haubitze (Schneider)" is mentioned and also that 12000 15 cm Gr. 14 and 6000 15 cm Gr. 12 per Month should be produced for these howitzers.
The diameter of the driving band of both shells with russian driving band was 157 mm while the shell had only 148,7 mm diameter. This would lead to 3,7 mm "air" in a 152,4 mm barrel which is to much in my opinion. For comparsion the same shell for french 155 mm long guns had a diameter of 154,1 mm so only 0,9 mm "air" in the barrel.
Again another document is mentioning a "russische 15,24 cm Kanone (200 Pud.)" which used the same two shells but a "Reibzündschraube" (friction igniter) is mentioned for this gun so it used no cases.
The last thing I have found is a drawing named "Skizze der Kasten für Hülsenkartuschen der russ. s.F.H. in s.F.H. M.10". Even with German as my native language I don't have the slightest idea about what this "russ. s.F.H. in s.F.H. M.10" should mean. Maybe it's a drawing about the adaption of a box for the russ. s.F.H. to a different howitzer the "s.F.H.M.10". The s.F.H.M.10 sounds much like the Schneider howitzer. Your case could fit into the box. The case meassurements are not given but wall thickneses and length of the box. It calculates to 213,5 mm free length and also base diameter of 167,6 mm are given. I think that's the best track which I have found
Sorry I don't have a real proof for it but I think your case is for this Schneider gun as indications are very good. I can look into the russian documents an other day (my day is over for now

)
Regards, Stefan