Two of my most powerful 'tools' are 'location' and 'contrast'. In the 'calendar' of the day GD15 is located in the middle, at noon. Earlier, in the morning (in contrast), sun is visualized as 'eating' = 'growing'. His face must be illustrated seen from the side to show the open mouth. The system of rongorongo glyphs is basically a system of signs written in 'silhouette' (though not with black inside the perimeter).

Fully grown, at 'noon solstice', the sun is shown as GD15. We will now test this hypothesis: In Wednesday he is still eating (face shown sideways) in Saturday he is 'satisfied' (a word which very much associates to Saturn) and shown 'en face':

That he in Saturday is close to the 'horizon' (limit) of the week may perhaps be visualized by his right foot being swollen - things near the horizon become large (a new guess to test).

In the Mamari moon calendar there is only one example of GD15 and that one is located at the start of the month, showing a clear sign of 'eating' = 'growing':

The remaining 'tests' will be found at this hyperlink.