lowercase letters ... form the plural with an apostrophe and an s". The Oxford Style Manual on page 116 advocates the use of common sense. vs states on page 602 that The 15th edition of The Chicago Manual of Style states in paragraph 7.16

How often have the words lowercase letters ... form the plural with an apostrophe and an s". The Oxford Style Manual on page 116 advocates the use of common sense. 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 )