I just noticed something - pencil lines. One runs vertically through the cartouche, it seems to begin at a dot and goes up through the 4 glyphs above the cartouche. The other line runs horizontally through the cartouche. And notice where the crosshair falls, right by a dot !
Get out your ruler guys, the vertical line runs through the center of the cartouche. If you lighten the pictures too much the lines won't show.
The vertical line cuts through the tops of the legs on both chicks in the cartouche. These lines had to have been used to layout the drawing.
There appears to be another line running horizontally through the sieve, but it's not straight.
Now if these marks were drawn at the quarry, they wouldn't have needed sight lines. But the lines would be helpful if drawn in situ.
Here is a pic from PBS.org, it was posted in 1997, not a good pic but I can see the horizontal line. I don't have good photo software, maybe you can enhance the pic to bring out the lines.