Ideas:

1. This strange glyph is - according to Fischer - carved on the edge of the tablet, not belonging to either side a or side b. But from the part to the right we can see that it should be placed here, as glyph no. 5 in the '2nd period'.

2. Another glyph, on the other side of Mamari, seems to be the same sign as that to the left in the glyph above:

          

3. Is this sign perhaps part of the sign for the 'sun'? The 5th glyph in a period should be a glyph with a 'sun', e.g.:

       

4. The sign might be the circular disc of the 'sun' combined with something else. But I do not believe this something else is the upwards 'flame' of the 'sun'. Rather I believe this something else to be a sign similar to the following:

    

But with 1 diamonds instead of three. Is this a period? It looks as if the glyph with 1 diamonds is being born from an egg, which explains why we cannot see more than half of it yet.