In 1965 the 850 was introduces during the Geneva salon d'Automobile. Swiss magazine "Automobil-Revue"devoted various articles to it in the salon catalog. A couple of photos:

850 op Fiat-stand Geneve1965

click on the photo to enlarge (opens in a separate window)

A few articles preceded that introduction, including a prototype. That started with a 600 with an extended rear:

Prototype 850

fiat 850 prototype 1

fiat 850 prototype 2

A four-door version was also tried. Maybe for the Simca 1000?

fiat 850 prototype 3

Italian magazine Quattroruote published this idea in the February 1964 issue:

quattroruote 02 64

Close. But no cigar.