That was hard work, but I've maybe found it. The puzzle photo was one of an auction lot sold in Germany, and depicts the Circuito di Modena held on 30.6.46. I have no caption for the photo, but I reckon it must be Franco Bertani winning the 1100cc sports car race.
According to "a certain site" this was a Stanguellini Fiat, but I thought they were only semi-streamlined in that year.