I have two possible shapes, and two possible highlighted letters.
The version where a vowel is highlighted gives an area that is associated with the bottom row but is not quite the 56a- rather the opposite.
The version where a consonant is highlighted gives an area that is the “correct” answer to the 56a as stated in a work with 28 letters in the title, but also there are 2 ambiguous versions of the shape.
I suspect it is the first and I am overthinking the lore.