I never seen a bronze coin with the Dadao (broadsword) shape like you claim. Dadao would have been impossible to create metallurgically in the pre Qin eras.
Furthermore, there is no archeological evidence that Dadao existed, from remains to drawn pictures. China records and preserves its history quite well, and there's no evidence of dadao-broadswords in early periods. The daos are two handed, with long stick like handles, with long straight blades or curved at the end.
The whole image of Chinese swordmanship portrayed in media using broadswords and Tai Chi Jian is pretty wrong, when in fact, dynastic Chinese from Han to Ming fought with swords that are not dissimilar from their Japanese counterparts.
There is a scene in Red Cliff, I think it was Zhou Yu, who had this long, two handed straight dao which he used to chop the legs of the cavalry rider to take the horse down. (No actual harm on real animals during shooting btw). That's more or less, classic portrayal how such weapons Sung elite troops take down mounted riders, though such a portrayal are centuries too early in the Three Kingdoms era.
