Ideas:

I think this glyph shows Full Moon, the shape of the moon and an additonal triangular sign in the middle.

The turnover from Waxing to Waning Moon, which occurs at the time of Full Moon, is in the monthly calendar of Mamari illustrated as if there are two adjoing staffs (which I suggest represent the periods of Waxing and Waning Moon). 

Perhaps these adjoining staffs are shown as the apex in the triangle of this glyph?