This coin has been identified to a reasonable certainty, to be a Kāi-Yuán Tōng-Bǎo. The coin is of a cash type, estimated to be 1 cash value. Coins of this type were issued during the mid-late Tang Dynasty (621-907 CE), from 732-907 CE.
The coin is made of cast iron, with a diameter of 25 mm (0.984 in). Its original weight is unknown, due to oxidation (rust) lightening its mas, over the centuries. Its rim is a conventional type, being relatively wide and raised. The central hole is a common type, with a square hole. The hole's rim is relatively thin, done to make way for the coin's Hanzi characters.
Observe
The coin's characters are read from top-bottom and left-right. They read "開" (Kāi),元 (Yuán), 通 (Tōng), and 寶 (Bǎo). The Hanzi were based on the writing of politician-calligrapher Ouyang Xun (557-641 CE). The practice of using the writings of famous calligraphers and officials was a common practice. Such as Emperor Huizhong's Slender Gold Script, being used in the Dà Guān Tōng Bǎo coin's design.
Notably this design is among the most influential of East Asian currency. Being the first Chinese coin to be valued on its inscription, rather than weight or material. In addition to being the first coin to include the inscription 通寶 (Tōng-Bǎo). Which was repeated by successive cash coin generations. Finally ending with the Indochinese Bảo Đại Thông Bảo, 1940.
Reverse
In regards to the reverse, this example is blank. Although some examples have a small up-ward facing crescent moon charm, located near the top. This shallow crescent moon can be sometimes difficult to spot, due to weathering. Or in the case of unclean coins, filled with compacted dirt.
Size Comparison
Additional Notes
Historically this particular coin was produced in 3 mints. Located in modern day Hubei and Sichuan providences. Which produced iron coins due to a lack of copper for bronze.
Due to a iron's relatively low durability, iron cash coins are often found in a corroded state.