I found a number of interesting information about this vehicle. I believe they are reliable as they come from the current owner of the vehicle (for sale by the way).
Engine is indeed Fiat 1100, gearbox from a Fiat 508, modified Bugatti rear axle, wheels from a Bernardet scooter.
Order passed in February 1952 from Benedictine to Albert Leblond in Paris for 1,936,000 French Francs and delivered in June of the same year.
Luciano