I've found it, whilst looking for details of another marque. It was built by Andre Pradeau of Limoges. Even the "certain site" does not have any further details, although it is known that one of his cars was built to the 500cc Formula 3 regulations.
The site with the puzzle image does identify it as a "racer 500 Pradeau monoplace", so they must consider this car to be his F3 contender.