You're familiar with five phase theory? Each of the five "elements" is associated not only with a particular directions and animal but also a colour, a smell, a taste, a sacred mountain, a race of barbarians, a fruit, a grain, and so on and so forth. So "five" comes to express "all sorts of". (Cf. "seven seas" in English.) Thus 五金 or "five metals" really covers all the sorts of metal known to us and--by extension--objects made from them.