AMD Duron 700
A somewhat uncommon AMD Duron 700 because of it's low vCore. It only needs 1.4V where most Duron 700's were shipped out with a 1.6V requirement.
AMD started with the first Duron's at 600MHz that often needed 1.5V or 1.6V. As the clock-frequency went up the need for a bit more vCore also went up. Most 950MHz or slower models need 1.6V or 1.65V. A few exceptions however since the best samples of a CPU-core need less power to operate and at the time AMD apparently tested their chips to see how low they could (safely) go.
Regarding the 'best samples': being able to use a low vCore (within the same line of chips) is in a way identical to overclocking. The best samples can usually run with a lower vCore or run at a higher clockfrequency with the, for that line of chips, regular vCore. In manufacturing a CPU the chips that are sliced from the inner circle of the wafer are usually better than those on the outside. When running either a low vCore or higher clockfrequency (with increased vCore) you'd want the chip that came from the inner circle of the wafer .
Of course: overclocking (and undervolting) is still a lottery. Also the architecture and manufacturing process of a chip will determine how it will overclock. Some architectures can handle an extremely low or high vCore better than others.