Four Chinese characters - Khai Nguyen Thong Bao (Vietnamese reading) / Upright crescent above the hole. 25.5mm, 2.77 grams. ca.1100-1500 AD.
A beautiful example!
The Vietnamese copies of the Chinese Tang dynasty Kai Yuan cash coins were probably produced for hundreds of years by both the Kings and the local lords. The Chinese Kai Yuans were very popular and circulated for hundreds of years after they were minted. They were widely imitated in Japan, Central Asia, Vietnam, Indonesia and Malaysia. The Vietnamese copies range from very crude to near-original. They can be distinguished by the unorthodox calligraphy or unusual casting. The Vietnamese Khai Yuans cannot be attributed to any region or ruler with any certainty - there are no detailed studies of the Vietnamese Khai Yuans, so these coins are still poorly understood.