Headstamp Identification Question


I have a shell with St (double lines) on the top and GFSP 199 on
the next line.
Then Sept 1915
and then in the middle at the bottom sf197
The shell is 9" tall and 3.5" in diameter.
Can anyone help me identify it and the meanings of these markings.
Couldn’t din GFSP in the list on the association website.




I think you have a German shell case for the 7.7cm C 96 n/A (Construction 1896 neuer Art) This field gun case is 77mm x 230mm R.
“st” means "st