The mid-to-late 70's were a busy and exciting time for the Italian company.

Fiat Trattori S.P.A had just been formed in 1974, counting four factories domestically, including the SPA factory in Turin, and the OM factory in Brescia, not forgetting four factories abroad, and six construction and assembly licenses. All having to be organized under the new company structure. 
The millionth Fiat tractor rolled of the line in 1974 as well. 
In 1975, Fiat went on a buying spree, acquiring a 20% holding in Laverda, and buying four factories from other Italian manufacturers, including the Lamborghini Trattori factory of Cento, and the Agrifull-Toselli company of Ferrara. And again in 1977, buying the Spanish tractor constructor Motransa, and Fiat's first holding in Hesston. All while developing and introducing a full line of new 80-series tractors.
In 1977, one out of ten tractor sold around the world was a Fiat tractor.

By 1979, all 80-series tractors had been revealed, and the series was such a leap forward in design and technology, that it made the rest of Fiats tractors look old in comparison. The new and modern square-cut lines reduced the appeal of the rounded styling of the Golden Ribbon models, which at the beginning of the 1980's, still made up Fiats small and basic entry-level models. Fiat therefor invested 35 billion lire in heavily updating and restyling their entry-level tractors and Vigneto/Frutteto versions, resulting in the 70-series in 1978, which got a facelift and expanded lineup with further upgrades in 1982, now marketed as the 66-series.

The 66-series was sold internationally and domestically as Fiat. Apart from France and North America, where it was sold under the Someca and Hesston brands respectively, with the North American Hesston 66-series being offered in a special "High Clearance" model for operating in tall crops. 

The 66-series like the 70-series carried over the strong and proven stressed chassis design from the Gold Ribbon series, using the same, or in the case of the 566 and 666 slightly modified, engines from the 500 and 600. The 766 arrived later in 1984, with the engine from Fiats 780.

The standard gearbox available for the 66-series, was an all new fully synchronized 12 forward and 4 reverse, with the option of a 20 forward and 8 reverse creeper transmission, or a 12+12 shuttle unit. 
The later being the first of its kind in a Fiat tractor, which prompted Fiat to give the "H" designation to tractors with this transmission.

New linkage and draft control, together with oil bath disc brakes and other new mechanical features, not only made the entry-level tractors from Fiat advanced, but also cheaper to manufacture, do to many of the parts being shared with already fully developed 80-series tractors, in turn making Fiat able to price the 66-series very competitively.

The 466 and 566 used a three cylinder, watercooled, direct injected 8035 diesel engines. 
With a displacement of 2.590cc and 2.750cc, the 466 developed 54 hp at 2.600, and the 566 developed 58 hp at 2.650 rpm, with 170 nm at 1.400 rpm and 200 nm at 1.400 rpm respectively.

The 666 and 766 used a four cylinder, watercooled, direct injected 8045 diesel engines. 
With a displacement of 3.456cc and 3.670cc, the 666 developed 68 hp at 2.500, and the 766 developed 78 hp at 2.440 rpm, with 229 nm at 1.300 rpm and 262 nm at 1.400 rpm respectively.

Fun fact. - The 66-series was the first full series at launch, to carry the "Terracotta" colors world wide. Fiats color change is said to have happened for two reasons.
The first do to new environmental regulations, meaning the orange color was labeled as "toxic" during painting and disposal of its residue, as the orange pigments contained comparatively high levels of lead and chromates.
The second reason was business strategy. Fiat was slowly moving towards a "one brand" strategy for the company, after having acquired Laverda and Hesston. Inspired by Italian "terracotta" red clay ceramics (meaning "baked earth"), Fiat decided to use red clay as their theme, as red clay in its natural state, depending on conditions, could appear like the orange colors of Fiat, or the red colors of Laverda and Hesston. With Fiat having proclaimed its "marriage to the land" before, it seemed fitting, that the final color would be given a deep and earthy tone, and named "Terracotta".
The color would be fully implemented in 1983, with the introduction of the "Fiatagri" brand.