Four Chinese characters Kai Yuan Zhong Bao ("The Inaugural currency", with the jing component of Kai does not touch the hole, hooked left shoulder of Yuan) / Nail mark above hole. 25mm, 3.96 grams. Issued in 621-718 AD. Schjöth #312ff; Hartill 14.2a. SKU T1789-49129
These are the later of the "early issue" coins. They probably date from the mid-7th century to 718 AD.
A crescent-shaped mark is often found on the reverse of Kai Yuans. The legend is that the Empress Wende (or, as in some folk legends, Wu Zetian) inadvertently stuck one of her fingernails in a wax model of the coin when it was first presented to her, and the resulting mark was reverentially retained. More prosaically, they appear to be a control system operated by the mint workers.
The Tang Dynasty, with its capital at Changan (present-day Xian), the most populous city in the world at the time, is generally regarded as a high point in Chinese civilization - a golden age of cosmopolitan culture. Its territory, acquired through the military campaigns of its early rulers, was greater than that of the Han period, and it rivalled that of the later Yuan Dynasty, Ming Dynasty and Qing Dynasty. In 907 the Tang Dynasty was ended when Zhu Wen, now a military governor, deposed the last emperor of Tang, Emperor Ai of Tang, and took the throne for himself (known posthumously as Emperor Taizu of Later Liang). He established the Later Liang Dynasty, which inaugurated the Five Dynasties and Ten Kingdoms Period. A year later the deposed Emperor Ai was poisoned to death by Zhu Wen. This coin is unconditionally guaranteed to be authentic.