In several photos I have of Japanese tea houses, there is the usual
hibachi in the centre of four tatami, with a tetsubin suspended over
it. The strange thing is that the tetsubin hangs from a large
J-shaped "hook", which appears to be some sort of wooden box. It's
enormous ! About 18" long and several inches thick.

Can anyone please tell me what this thing is called, what it's for,
and why it's the shape it is ?

Thanks