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 vs after noting that the usage has changed

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 after noting that the usage has changed occurred in a corpus of books? (source: Google Ngram Viewer )