The question mark is because perhaps a few glyphs are destroyed in the 16th of the 20 glyph sequences, at least according to Fischer who has marked two places with three dots, which should mean that there is an unknown number of glyphs missing:

 

According to Bathel, however, who always tried to indicate the space of destroyed glyphs, the first place has no missing glyphs. The second place perhaps has room for one glyph, but that presumes that neither the glyph to the left nor the one to the right stretches into the missing space. Mixed glyphs may take large space.

 

As 208 / 26 = 8, I believe that either the glyph to the left or the one to the right (or both) stretches into the vanished text space.