A good clue would be the diameter of the neck. Would it be about 77mm?
The font of the lettering and numbering appear very German.
St = Strengthened case (Stark)
HL = Haniel Luege Düsseldorf (brass factory)
MAI 1917 = May of 1917
https://www.pritzkermilitary.org/exp...oclc/944444048