Ideas:

1. This glyph, in period no. 31,  is not identical with the earlier two (in the 4th and 18th periods of the calendar). We can compare the three glyphs:

The third glyph, that in period no. 31, has a prominent bulge to the left in the sign for the double-month at the bottom. What does this mean? It must be a sign.

2. That the first two (in the 4th and 8th periods) are identical possibly means that there is no intention of the writer to distinguish in these two glyphs between the first and the second half of the year. Another possible explanation is that both these two represent the first half of the year, and that the second half of the year is described in the third glyph. A comparison with similar glyphs in the rest of the text in Small Santiago Tablet shows that the writer is consistently writing two variants.