There is no evidence upon any coachbuilder who has dressed the Fiat 508C chassis in this manner. Hardly be assumed that it happened in the factory. Also this is not a special body for the Mille Miglia. After all the car benefits from all the innovations that are integrated in the series until 1938.
This includes the engine, now with overhead valves and 1100 cm³ and also the gearbox with now (except the first) three synchronized gears. The rigid front axle has been replaced, too. In general the chassis scores well in tests. In the U.S., where the car was photographed, a maximum speed of 80 m/h is specified, which speaks for the aerodynamics of the body. There is also the fact that the indicated horsepower was probably determined by the CUNA standards, thus without exhaust tract etc.
By the way there is an even more famous car on the base of the 508 C at the same time. This is developed from Ferrari and his team in duplicate, but not under his name and with a much more powerful engine. If you want to compare, you have the oppurtunity here. 08/09