I sent the photo of the shell casing to my eldest son who lives in Germany, and does a bit of collecting/buying/selling himself, but predominantly medals, although he has quite a bit of WW1 stuff in a cabinet.
He has his deceased German father-in-law's WW2 Luger pistol, his father-in-law was in the S.S., but my kid keeps that on the down low, he also has a WW1 pistol, (maybe a Luger, I don't know), which was my German daughter-in-law's grandfather's.
My kid said that H.L. was Haniel Luege, the manufacturer in Düsseldorf, the H.L. 8 was probably an inspection mark, H.L. 50 may be the lot number, and St at the top might mean strengthened.
I didn't ask what kind of piece would have fired it, I'll email him again.