Never mind Cees, (you're probably too busy to share those details you were talking about anyway), but now the details have come out first-hand from Wolfgang Matt. Dieter e-mailed him and got the correct story. It wasn't Genk, and it wasn't 1963. Here is what Wolfgang told his friend Dieter, who shares them with us:

This picture was taken by Mr.Streil in 1965. Ralph Brooks traveled directly to Liechtenstein to visit the Matt family after he won his second World Championship in Sweden . The picture was taken on the Liechtenstein airfield used by the family. This Orion belonged to Ludwig Matt, the father of Wolfgang Matt.The young man is Wolfgang Matt. The balding man on right side was a member of local Liechtenstein Club. The other 2 guys Wolfgang couldn't remember; they were apparently American friends of Ralph Brooks. The plane later crashed due to radio failure and was destroyed.
You can see the Club area of the Liechtenstein field in the background. Today this building is closed, but it has the same outside lines as then…

It’s a high quality historical photo…