"Copper-wheel" as in Tiantai terminology

[FromTheEarth]

Today a forum member referred to an old thread in this sub-forum, i.e., viewtopic.php?f=59&t=23994.
In that thread, there was confusion around the term "copper-wheel" used by Nichiren in his writing. Instead of replying to the old thread, I think it might be better to create a new one.

The terminology originally came from the 菩萨璎珞本业经/Bodhisattva keyūra mūla karma sūtra/Keyūra sūtra, which compared the fifty-two stages of bodhisattva practice to different kinds of kings. It was then adopted by Ven. Zhiyi and became a part of the Tiantai doctrinal system.

Iron-wheel turning kings refer to the first ten stages; copper-wheel the second ten; silver-wheel the third; gold-wheel the fourth; vaidurya-wheel as the forty-first; and, the Cintāmaṇi/maṇi, the forty-second, which is synonymous with Buddhahood. For reference in Chinese see this.