Ideas:

According to Zehren people used to talk about the disappearance of the moon, at the time of new moon, by explaining that the moon was temporarily confined in a sack, in a coffin, a basket, or an urn.

Is there perhaps some thinking like that behind this glyph?

Though probably there are other thoughts involved too, where focus is put on the New Moon as the origin of the coming Waxing Moon. We could compare the dark period with a period of gestation, a time of mystery when new life is created.

In the uterus of a woman a period of nine months is needed for creating a new human. A new moon is created in two or three days. Neither the fetus nor the unborn moon are seen until the time is ripe.