It's a 7,5 cm IGr. 18 Üb Al, which was shot with the 7,5 cm le.i.G 18 and le. Geb. I.G. 18.
The 7,5 cm IGr. 18 Üb Al is a practice shot, the reduced HE charge had aluminium powder mixed in, which resulted in a bright white flash upon impact.
Best regards,
Zünder