There is some difference of opinion regarding the use of apostrophes in the pluralization of references to letters as symbols. New Fowler's Modern English Usage
states on page 602 that The 15th edition of The Chicago Manual of Style states in paragraph 7.16
How often have the words There is some difference of opinion regarding the use of apostrophes in the pluralization of references to letters as symbols. New Fowler's Modern English Usage and states on page 602 that The 15th edition of The Chicago Manual of Style states in paragraph 7.16 occurred in a corpus of books? (source: Google Ngram Viewer )