Excelent find!
The steel screw leads me to wartime use. in this case You can choose from these projectiles:
10,5cm Aufschlaggranate M 35,
10,5cm Doppelzündergranate M 35,
10,5 cm Aufschlagzündergranate 35 (t) FEW,
10,5 cm Doppelzündergranate 35 (t) FEW,
10,5cm Aufschlagzündergranate 325 (j) -jug 36-,
10,5cm Doppelzündergranate 326 (j) -jug 36-,
10,5cm Halbpanzergranate 327 (j) -jug 36-
10 cm Panzergranate rot,
10 cm Panzergranate rot (Ei)
10,5cm Hl/B Gr. 302(f)
10,5cm K Hl/B Gr 39 (f)
10,5cm K Hl/C Gr 39(f)
10,5cm Lt Gs
I guess the German AP is the easiest to find.
Sources: HDv 119/401, HDv 119/405, HDv 119/417, D 435/1
Bob