In the early 60's, Fiat launched an effort to streamline their lineup, with the intent of making their tractors easier to market and service, thereby strengthening their position internationally.  

Fiat tractors had a reputation for being reliable, easy to maintain, and well priced. But not exactly for being or having a cohesive and original design language. For the replacement of the 100 series, Fiat therefor decided to hire the help of non other than Pinninfarina, the studio behind some of the most beautiful cars in history, and for many years Ferraris preferred designer and coachbuilder. 

Introduced in 1965 the new, and officially Fiats first series, would be known as the "Diamond Series".
Two years after launching the series, Fiat introduced the 715 model. Becoming the largest tractor available in the series. 

The tractor was sold domestically under the OM brand, and internationally as the Fiat 715.
Though in France, it would only be sold as the Someca 715. Someca being the agricultural arm of Fiats French subsidiary Simca.

The 715 was constructed using a heavy box section steel frame. In one end attached to each side of the gearbox and differential casing, in the middle carrying the OM developed engine, and the other end attached to the front axle.

The gearbox was a seven forward and two reverse, and had PTO and locking differential on the rear axle as standard, with the three-point hitch system being Fiats advanced "Control-Matic".
The 715's gearbox didn't come with synchronized gears like the 315 and 415. Instead the 715 offered the mechanically activated on-the-go reduction gear named "Amplicouple", later simply known as "Automatic", as an option for the 715. 
With results similar to later and more advanced hydraulically and electrically activated powershift systems we know today, the Amplicouple/Automatic system would act as a quick engage and disengagement of the reduction gear in any gear, preventing the driver from having to come to a stop to engage the reduction gear, when momentum is most needed.
The reduction gear was "armed" by a leaver to the left of the driver. Power was sent through the PTO shaft, and could then be returned to the gearbox through the reduction gear on command via the clutch pedal. This reduction gear was only ment for emergencies though, as its engagement would wear out clutch and throwout bearings prematurely.
This coupled with the extra cost, ment that most costumers didn't order the Amplicouple reduction gear system.

Another advanced feature of this tractor, was the use of disc brakes. A very advanced technology for the time, rarely seen on other than high-performance cars of the time.

The 715's engine was the same as in the 615, though by increasing peak rpm and fuel delivery, power output was increased from 66 to 75 hp.
But not long after the 715's launch, OM and Fiat would increase the OM CO2D engine's bore and stroke considerably, upgrading displacement from 4.397cc to 4.940cc.
The new engine was bigger and less stressed, and though it only gained three horsepower, the torque figures increased dramatically, properly distancing the 715 from the 615.
715's running the larger engine were designated 715L or 715-5L, showing the new engines displacement of (almost) five liters.

The Diamond Series enjoyed considerable success, being recognized for their new and modern design, performance, price and reliability. Production at Modena rose quickly from 34.088 tractors in 1964, to 41.156 in 1966.