Jack,
I think you are talking about the Fiat Derivazione Abarth 750?
I doubt that these cars ever left the Abarth factory without any badges.
All the pictures I have seen of Abarth 750's show differences with the ordinary Fiat 600. You can recognize them by the badges (front grill), wheels, engine and some of them also had paint striping on the sides or front bonnet in a different colour than the rest of the body. I recon these factory-750's also have a data-plate fitted in the engine compartment near the Fiat data plate.
Than again you were also able to buy Abarth-kits in those days. These kits consisted of the same parts as the ones fitted in the factory, but these cars lack the data plate.
I am not sure if the engine blocks were also numbered by Abarth???
It's hard to recognize an original Abarth from an ordinary fiat 600. Features were: different air filter, carburettor, camshaft, crankshaft, pistons, air-duct for radiator, exhaust, Fergat wheels or hub-caps and an Abarth front grill.
I hope this helps,
Alex van de Wetering
