We can see that the vertical divisions are across the shell, whereas the horizontal line is disturbed by the part with two marks inside.

Immediately to the left of the central vertical axis there are 11 + 11 = 22 marks and to the right 16 +17 = 33. And 2 + 22 + 33 = 59 = the number of nights in a double month.

Or we could count 2 + 2 + 11 + 11 = 26 to the left of the central line and 33 at right.

26 ↔ 260 days together with 33 nights from the breaking of the coco-nut to the solstice.