This depicts, on a dotted circle, a tone mark  used in the Tai Khuen for words that have not developed regularly from Proto-SW Tai and therefore cannot have their tone shown by the 2-mark system.   It is a borrowing of the Thai tone mark mai tho and is therefore a doublet of Tai Tham sign Tone-2.
I created it using the Lamphun font, which I derived from the Hariphunchai font, for the dotted circle and the Kinnari font for the tone mark.
Ed Trager produced the base glyph (dotted circle).  The copyright in the tone mark lies with one or both of Db Type and the National Electronics and Computer Technology Center, a Thai organisation.