FYI, I have corrected my original post. I thank Kyle DeJute for her knowledge of UEB-Technical.

The reason a grade 1 indicator is NOT needed for ::/128 is because when we get to the slash, it becomes impossible that the preceding cell of dots 25 could represent any contraction, and once that is established, it then becomes impossible that the first cell of dots 25 could be any contraction [because both the “con” and the “cc” contractions must be followed “by a letter, a contraction, a modified letter or a ligatured letter”].