My guess is that if your measurements are correct it is a shell for the 9cm C73/1873 Feldkanone.
As I understand it the bulk of German artillery during WW1 were 7.7cm, 10.5cm or 15cm guns however just after the Franco-Prussian war of 1870-73 they did produce a 9cm (Actual 8.8cm) piece known as the 9cm C73/1873 Feldkanone (there were also updated models designated as the C73/1879, C73/1888 and C73/1891) which saw service through to the end of WW1 with the German Artillery.
However that being said we do have a lot of artillery personnel on these boards I am sure one of them will know a lot more than I do on this issue.
PS. If there is a %Cu or a % anything somewhere on the shell it can be used to determine the purpose of the shell itself.