Ideas:

1. If we compare this glyph with the similar one in Wednesday:

we can see that they are not identical. There is one set of closely placed internal downwards sloping marks in Wednesday, but two such sets in Thursday. Is this another sign of the thunder twins?

2. In Wednesday the closely placed marks are located in the middle, but in Sunday, where there is also such marks, they are placed at the top:

In Thursday the double set is at the top and in the middle. Some meaning surely lies in these locations of closeness.