welcome to the forum.
your cartridge case is for the standard German fieldcanon of that time, the 7,7 cm Feldkanone 96 n/A.
The case was made in May 1917 as you thought but not on the 29th.... the "295" is the lotnumber of that batch of cases which were indeed made by Polte in Magdeburg.
The "St" (=Stark=strong) means that the case was strengthened (sp?), the first cases for this canon (until ± 1905) had thinner walls.
Sp406 is the control/inspection mark of the manufacturer.
Enjoy your stay on this wonderful forum