This type of glyph appears in Monday, Wednesday and Friday. Therefore I believe that it might be indicating that at certain times the 'planet' disappears from sight.

Evidently this is true for the moon, which once a month disappears - at the time of new moon.

Venus sometimes disappears for 8 nights in a row (when changing from evening to morning star), and at other times (when changing from morning to evening star) for 40 nights in a row.

Mercury also disappears, more often so than Venus. We need double signs in Wednesday to show this.

Reference: Zehren