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 as

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

as

noun
  • Any of several coins of Rome, coined in bronze or later copper; or the equivalent value. 

  • A libra. 

conj
  • used before a preposition to clarify that the prepositional phrase restricts the meaning of the sentence; specifically. 

  • At the same instant or moment that: when. 

  • Being that, considering that, because, since. 

  • Varying through time in the same proportion that. 

  • In the (same) way or manner that; to the (same) degree that. 

  • Functioning as a relative conjunction, and sometimes like a relative pronoun: that, which, who. (See usage notes.) 

  • At the same time that, during the same time when: while. 

  • Expressing concession: though. 

  • Used after so or as to introduce a comparison. 

  • Used to introduce a result: with the result that it is. 

adv
  • Considered to be, in relation to something else; in the relation (specified). 

  • To such an extent or degree; to the same extent or degree. 

  • Used to intensify an adjective; very much; extremely 

prep
  • Introducing a basis of comparison, with an object in the objective case. 

  • In the role of. 

  • by way of 

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 as occurred in a corpus of books? (source: Google Ngram Viewer )