BTW, forgot to mention - it is a "Low G" note when sounded naturally with the bore unrestricted. It is 'coaxed' down (by impedance loading) to 'Low D' - it can be microtonal, or even glissando (slide). It gets a bit more tricky with the overblowing octaves - if you can play a Tabor pipe, you can handle the fact that you only get half a scale on each particular overblow pressure. On a normal whistle, and this is no exception, each successively higher note also does require a slightly stronger breath pressure to voice correctly.

You definitely need a well controlled diaphragm and a good set of ears to play it.

Robin