This ancient Chinese coin is a Wu Zhu (五銖, Five Shu ) issued during the Western Han Dynasty (202 BCE-9 CE). The Wu Zhu are noted for having history the longest mintage in world. Spanning 736 years from 118 BCE-618 CE. They created during the 3rd era (元狩 Yuánshòu) of Emperor Wu (156-87 BCE), as a replacement for the previous San Zhu (三銖, Three Zhu) and Ban Liang (半兩).
Due to their long mintage, roughly 900 types and 1,800 variants of Wu Zhu have been documented. This includes unofficial variants produced under the reign of Emperor Wang Mang (45BCE-23 CE). Who during the short lived Xin Dynasty (9-23 CE), abolished the Wu Zhu during his second monetary reform (9-10 CE).
The popular Wu Zhu secretly remained in use, even under the threat of death. After the overthrow of Emperor Wang Mang, the Wu Zhu was reintroduced. Remaining in use under the Eastern Han Dynasty and their successors. Until the introduction of the Tang Dynasty Kāiyuán Tōngbǎo (開元通寳), which remained the standard from 621–907 CE.
This particular Wu Zhu example appears to be an early variant, possibly cast in 115-113 BCE. The coin's rounded edge points to it being a second series coin. As the initial series of Wu Zhu featured an unfiled edge. Causing some coins to have undesirable flanges and sharp points. The coin is made of cast copper, with a diameter of 25.3 mm (1.00 in) and a thickness of 2.1 mm (0.08 in). It's weigh is 4.59 g, slightly of it's reputable "Five Shu" weight. Meant to be equal to 500 gains of millet, with 1 Shu equaling to 0.8 grams.
Observe
The observe is relatively sparse as many coins of the era. It feature an early form of Hanzi, in cast seal script. The symbols being closer to ideograms, than the later formal script seen in Tang Dynasty coinage. In modern Hanzi the symbols read 銖五, or Five (銖) Wu (五). Notably some Wu Zhu will occasionally have this symbols backwards and thus read as such.
Another notable point is the coin's outer rim, which is thin and irregular. Unlike most later Chinese coinage, which feature thick and well defined rims. With some exception such this cleaned example of a Song Dynasty Dàguān Tōngbǎo (大觀通寶).
The center hole is rimless and features edges that bevel downward. Notably this feature is common with most Wu Zhu. Although some later post-Han variants feature a thin edge. Usually toward the top and bottom of the central hole.
Reverse
The reverse is appears blank at first glance, although does some key features. The central hole features a raised edge. This feature was added to make it more difficult remove material from the coin. As removing the edge would show the coin has been intentionally tampered. Thus reducing its overall value.
Below the lower edge is what appears to be an imperfection. This mark is known as a "star" and is an intentional design. Some Wu Zhu can feature multiple "stars", some featuring as many as 5. Other feature include shooting stars, poetically known in China as the "tadpole lucky cloud". To more mundane "moon" and "sun" imagines.