Unusual concave based 1915 shell case

i need help to identify a recently aquired shell case please…it has a mouth diameter of 75 or 76 mm and is 692mm long. the base diameter is 93.3mm and it is concave to a depth of 6mm at the primer (something that i have never seen before)…headstamp is PATRONENFABRIK KARLSRUHE 1915 904 …perhaps the 904 holds the clue??? the closest possibility from my somewhat limited database is the Italian WW1 76/45 AA which has a case length of 699mm and base diameter of 94mm… can anyone offer any suggestions as to what this case may be and from what gun…it is very long and slender and therefore looks quite unusual…thanks very much…Charley King

Your case is the same as the Italian 76/45, which was a 1911 Schneider Naval Gun. This case was made by Germany for those Schneider guns in their possession or the possession of their allies. Italy was not among those allies during WWI.