This is a car which took part in the Maserati UK Race for Pre-1961 Grand Prix Cars at the Coys International Historic Festival meeting at Silverstone in July 1999.
It's a 1959 Ferrari Dino 246 F1 car, driven in the race by its owner Robin Lodge and appears to be serial number 0006. I understand that Ferrari, and in fact Maserati and other Italian racing teams, would often test racing cars on the public roads and a kind of temporary registration number would be stencilled on the car, as we see on the rear of this Ferrari - it's something like the trade plates we use in this country. The 'prova' stands for 'test', 'MO' indicated 'Modena' and the '31' is just a numerical identifier. Presumably it's on this car to show some kind of period authenticity.