Wow, that was the hardest puzzle I've tried to solve so far.......
I think I have the answer you need but only because I had downloaded a motorsport site some time ago dealing with the Rijeka/Opatija region circuits in Croatia and I eventually found the car on this site.
I don't speak Croatian and in my research was surprised to find that Fiat is Fiata in Croatian!
Anyway to cut a long story short, I believe this car was based on a Fiat Ballila (or Fiata Ballile in Croatian) and built by Dante & Luciano Saletnig of Rijeka. If I understand correctly the car was first registered in 1961.
That's enough for today and I'm going to put my feet up now and relax with a glass of wine!