Fig. 2: Ming Dynasty coin samples from the North Reef.

Sample Hongwu Tongbao was cast for the 1368–1398 (AD) period, and the average size and weight were as follows: 24.3 mm outer diameter, inner diameter of 6.7 mm, thickness of 1.4 mm, and weight of 3.8 g. Sample Yongle Tongbao were minted from 1408–1424 (AD), and the average size and weight were as follows: 24.8 mm outer diameter, inner diameter of 6.3 mm, thickness of 1.5 mm, and weight of 3.9 g.