This is word/letter puzzle. Take a hike over to Section 19.6 and use the guidelines there. The example given there does not match your situation exactly, but it can be generally used. Treat each missing letter likea worda in a vocabulary list and enclose the group in a box. Note that the box is mentioned specifically in the print. Write a TN to explain that the letter indicator and caps are not used with the letters. Also explain that a single hyphen is used to indicate each missing letter.