Before revision: Note: the letters "I", "O", "Q", "S", "T", "X", "Y", and "Z" shall not be used as modification letters For example: AN/PAS-13A and AN/PAS-13B are modifications of AN/PAS-13.
After revision: Note: the letters "I", "O", "Q", "S", "T", "X", "Y", and "Z" are not to be used as modification letters For example: AN/PAS-13A and AN/PAS-13B are modifications of AN/PAS-13.