Though the "Super series" received improvements, these tractors largely followed the design of their predecessors, and like them, were built to last. In 1954, the numbered or so-called "Hundred series" tractors appeared. The "Hundred series" models used numbers instead of letters to identify the model. The new models were given slightly different looks and a few new features, but were still essentially the famed "Letter series" tractors. The Farmall Cub continued unchanged, but in 1955 a new 'low-boy' version was added, featuring a shortened 62.5-inch wheelbase and a frame eight inches lower than the regular Cub tractor, which improved the machine's center of gravity. 1956 saw the introduction of the IH Model 350, which offered engines using a variety of commonly available fuels: gasoline, diesel, or LP-gas. The diesel engine version had a direct-start feature, and could be started and run using only diesel fuel. In 1957, IH again gave the tractor lineup an overhaul. Although the basic design was still not changed to any significant degree, new white paint was added to the front grille and sides, new engines were introduced, and new number designations were added. Along with these additions, the Torque Amplifier was added to the Model 300 and all larger models. This provided on-the-go shifting to suit varied needs; the operator was offered ten forward gears and two reverse, instead of the usual five and one. Another addition to these tractors was the independent ("live") power take-off (PTO), which meant that the farmer could run the PTO even when the clutch was disengaged (clutch pedal pushed down). Although the new tractors did improve sales, IH's innate conservatism and reluctance to update their tractor line in response to changing times was becoming apparent. At the Hinsdale, Illinois Testing Farm in July 1958, IH entertained over 12,000 dealers from over 25 countries. IH showed off their new "60 series" of tractors: including the first of their kind, large six-cylinder 460 and 560 models. But the excitement caused by the new introduction was short-lived. In June of that year, IH recalled the 460, 560, and 660 tractors after reports of mechanical breakdown in the field. IH, who wanted to be the first big-power tractor manufacturer, had inexplicably failed to substantially enlarge and re-engineer critical drive components on the new six-cylinder tractors.