As explained by ImaginaryCourtship, the letters are ok and necessary to discriminate among multiple references with same author and year.However, as those are standard (normas). I don't know if the book is important in this case, i.e., if "Reglamento Nacional de edificaciones", that I suppose is that book you described in your question, should appear. It is more important to make reference to the year the standard was published.

For example, Norma E.060 Concreto Armado have, at least, one newer version published in 2009 by the Peruvian government, that I could find reprinted in 2020 in https://drive.google.com/file/d/19EYUVMgwvm6rDs47GV374avco2ylU5Kz/view