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Below I profile some of the best-known rare copper issues collectors look for, with explanation and market context. 4.1. Kaiyuan Tongbao (開元通寶) — the Tang benchmark (but rare forms exist) The Kaiyuan Tongbao was first cast in 621 CE and became the model for later cash inscriptions and flan sizes. Standard examples are common museum pieces; however, rare variations — unusual calligraphy, mintmarks, and special large castings or high-grade archaeological finds — can command collector interest. The British Museum holds several representative Kaiyuan specimens and associated finds, which are useful references for identification and size/weight expectations. britishmuseum.org +1 Why serious collectors care: Kaiyuan Tongbao is the archetypal early medieval Chinese cash. Variant characters, unusual reverse marks, or an exceptional state of preservation make some specimens desirable despite the many surviving examples. 4.2. Song dynasty “Da Guan Tong Bao” large denominations a...