The * indicates that it was manufactured by Hirtenberger Patronen, Zündhütchen und Metallwarenfabrik. The "HIRTENBERG" indicates that it was manufactured in Hirtenberg. The C an 6 could indicate any number of things. Most likely, the C indicates that the round used cordite as a propellant and 6 is the date code. The absence of a caliber designation means it was definitely produced for military use. The off-center placement of the stamping leads me to believe that it was most likely produced during a time of war when quality control was less important than keeping up with demand. You could get in touch with HP and they could possibly give you an accurate age. I know that HP started stamping with two *'s at some point to differentiate the newer "clean" ammo from the earlier rounds stamped with one * due to Jewish prisoners being forced to manufacture the ammo during WW2.