Four chinese characters Kai Yuan Tong Bao ("The Inaugural currency") / Crescent abov the hole. 25mm, 3.16 grams. Issued in ca.618-907 AD. Schjöth #406 var (without the crescent); Hartill 14.102var (without the crescent).
The iron cons were probably produced in Hebei in Sichuan and circulated in that area. They are much more rare than the normal bronze Tang Kai Yuans.
The crescent (or "nail" mark) on the reverse has an interesting legend attached to it. According to these legends, the Empress Wende, when presented with a wax model of the coin stuck one of her fingernails into it. The resulting crescent-shaped mark was reverentially retained. The more prosaic explanation is that these marks were control marks employed at the mint.
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.