[Nicihntm]

I have an unfounded pet theory that this is what Buddha meant, rather than a manufactured disk of some kind.

My own idea is that they are Hindu meditations bent to the purposes of the Dhamma. This is how I account for the fact that despite being canonical, these sorts of meditations are also noted in the Canon to be (in one way or another) less beneficial than anapanasati, having limits on their usefulness in samatha practice and being wholly unable to support a vipassana practice.